A321 First Officer reported an abnormally elevated Aft Cargo Bay temperature during cruise. After conferring with maintenance; the flight executed a precautionary diversion for maintenance.

Narrative

Flight from ZZZ to ZZZZ; flying a route down to coast to avoid weather Severe WX. Between ZZZ1 and ZZZZZ Captain did a normal system monitor check. Upon looking at the COND page; he noticed the AFT Cargo Temp at 180 degrees (Steady Green); asked me if I had seen a temp that high; I replied No! We kept an eye on it; over the next 10 minutes; noticed it rising to 188 degrees; then 194 degrees (still Steady Green) where it stayed for the duration of the flight. This made us uncomfortable for normal Cargo Bay temperatures in my 3;000+ hours flying this aircraft at 2 different airlines have never exceeded cabin temperatures during cruise. We simultaneously started looking into what could be a catastrophic problem. I got into the QRH AOM/FOM/and other manuals to look for temperature limitations and fixes. I found none; anywhere. During the same time; Captain got on the Phone with Operations and phone patched to Maintenance Control; where they attempted multiple resets to no success. When Captain got off the phone with Maintenance; he said; 'Well nothing fixed It; and they (Maintenance) said we are on our own.'At that point; we agreed as a crew that we didn't know what was going on down there. We were both not comfortable taking an aircraft with this indication over water; with limited divert options to an international location; and with no company or Contract Maintenance at the destination not knowing what was going on down there. The indication was steady at this point; not rising higher [than] 194 degrees; nor giving any other indications to the back when we notified the Flight Attendants and told them what was going on. At this point; we felt we were not in a land as soon as possible scenario; but in a land as soon as practical scenario but keeping land as soon as possible options available and we decided to divert to ZZZ2. We gave ATC a PAN PAN call since we were going to a divert airport that wasn't our alternate but that we were not at the time an emergency aircraft; we just wanted to have all our bases covered. We notified Dispatch both via phone patch and ACARS. During descent we were cleared the ZZZZZ Arrival to ZZZ2.We were still grossly overweight with a lot of gas; so we referenced the overweight Checklist. With that; we decided that with the condition still not worsening from what had been continuously indicated with no secondary indications or issues coming from the cabin; Captain and I both agreed that in an effort not to compound the problem we should hold to burn down gas; but having immediate [landing] options nearby; so we held at ZZZZZ1 at 16;000 ft. for 30-40 min and got to a calculated weight that would put us on deck in ZZZ2 right at Max landing weight. We continued to ZZZ2 via the arrival; for an uneventful Runway XX landing. On a side note; the XX transition from the ZZZZZ Arrival is a bit of a slam dunk. We asked for a bit longer final to get down; stabilized; and situated; and ATC approach gave us a few last second altitude; headings; and speed amendments through final which started to task saturate us as we prepared for landing. For example: open descent from 6;000-3;000; we were told to amend altitude and maintain 4;000 ft. while passing 4;200 feet; thrust idle; and speed brakes out. We blew through the altitude and had to climb back up Auto Pilot off; ALT horn blaring; and engines spooling up to 80+%; and getting speed brakes back in. We were also given several headings through final; and were asked to slow to final approach speed 10 plus miles outside the final approach fix at 4;000 feet; and approach didn't give us an approach clearance until we had flown through the glide slope on the localizer; and had to join from above; in order to sequence aircraft landing [Runway] XYL; which started to compound our landing configuration planning; and a go around was not something we were hoping to do; especially off of a PAN PAN call. We later found that on a couple of occurrences; ATC did not pass the PANPAN or divert info from Center to Center to Approach. We were later reassigned upon arrival to fly a ZZZ3 Turn that left in 5 minutes.I told Company that we just went through a lot; and that if they wanted us to fly that turn; that I needed a break for an amount of time that I was setting; some food; and so the Captain and I went and had a sit down meal; decompressed; talked about what had happened; after an hour or so I assessed my fitness for duty and felt I was good to fly the turn. The Captain and I were sitting on the jet; and when we called for push going to ZZZ3; Ramp told us to hold and that Maintenance wanted to talk to us. The maintenance team member got on the radio; asked if we were the crew that brought Aircraft X in from ZZZ; I replied yes; and he told us we needed to fill out a fume report. Captain asked what he was talking about; and told him this was the first we have heard about any fume incident. The Maintainer on the radio said the Flight Attendants had filed a fume report after the flight; and we needed to do the same. The Flight Attendants never told us anything about fumes nor their intent to fill out any such reports. Captain then filed a fume report said he had no information to give for we knew nothing about it. I felt Captain had communicated very clearly what was going on to the flight attendants about the situation; and our intent to divert. They had never communicated any such issues during any duration of or after the flight. Unknown from the high temperature in the Aft Cargo Bay. We did later saw that Maintenance replaced the sensor in the aft cargo bay via the app. We were curious that with all the ZZZ team members in Training vests if maybe they stacked the bags in the aft cargo bay above the line and possibly covered the sensor or did something to It. However; As far as the 'suspected' fume event; there was no communication from the flight attendants to the cockpit crew for this event. We noticed via the app that Maintenance later deferred the APU to prevent 'further fume events' due to the suspected fume event we were later notified about from ZZZ2 Maintenance. Not sure why if there is a fume event (or even a suspected fume event) why they don't further investigate the problem or fix the problem; rather than just pencil whip it into deferment. If there was any fumes in this situation; how would MX (Maintenance) know they came from the APU and not from any potential problem we were encountering? Put quick reference limitations on a Cockpit Card or in the QRH or checklist to quickly reference for situations such as these. I felt like we were on an Easter egg hunt for something that we later found wasn't there which wasted a lot of our decision making time. I know that Airbus FCOM (Flight Crew Operations Manual) and other manufacture manuals/reference material has such limitations for quick reference from my experience at a previous Airbus airline. Why can't the company get on board with Airbus??? Make sure Flight Attendants communicate to the cockpit if they were smelling fumes.