A320 First Officer reported aircraft cabin altitude began to descend ahead of depressurization schedule on initial descent. The flight crew continued on programmed descent and and a normal landing at destination airport. The flight crew made a log book entry and maintenance ran their checks and signed the aircraft off. The Captain refused the aircraft after disagreeing with the maintenance actions.

Narrative

Late takeoff out of ZZZ1 due to Engine #2 HP Valve Inoperative. We followed MEL procedures for low power settings in descent. Descent into ZZZ at FL245; we started getting the altimeter flashing amber (wanting local altimeter setting). Both Captain and I checked the FMC; and programming was correct. We continued descent and set local altimeter at FL180. We were a little task saturated with getting ready for approach; moderate turbulence flying through buildups; unable to deviate due to traffic; TCAS TA ; and it was getting a little hard to hear in the cabin. At approximately 8 ft. 500 MSL; we get an ECAM (CAB PR LO DIFF PR...A/C V/S REDUCE). The aircraft was only doing approximately 1;300 FPM descent. Looking at the pressurization page; we see the outflow valve amber and fully open. FMC was correct with destination and data in approach PERF page. Since we were below 10;000 ft. MSL and about to land; we continued to the field and landed uneventfully. No one in the back knew of the issue as masks did not drop.We were scheduled to take this aircraft from ZZZ to ZZZ2. Maintenance performed checks; and all their tests passed and was going to put the aircraft in service. The Captain was not comfortable taking the aircraft as maintenance could not find a reason as to why the outflow valve fully opened. Captain refused the aircraft and I concur with his rationalization. The #2 HP Valve Inoperative would not have caused the issue; and given that we had a flashing amber altimeter on the PFD at FL 245; it did not feel right taking the aircraft with a plane load of passengers not knowing the cause.Subsequently; we flew the aircraft to ZZZ3 at 10;000 ft. MSL and prior to taking off we had a PACK 2 failure which we had to have deferred.