B737-800 Captain reported an air turn back after receiving an IAS DISAGREE on both PFD's.

Narrative

Conducted OE (Operating Experience) with NH (New Hire) FO (First Officer); FO was PF (Pilot Flying). On Departure SID; leveled off at FL260; in IMC; TAI (Thermal Anti-Ice) on. Shortly after reaching FL260 we saw IAS DISAGREE on CA (Captain) and FO PFDs (Primary Flight Displays). FO's airspeed read about 25 knots less than CA's. Ran Airspeed Unreliable checklist but could not determine which airspeed; if any; was reliable. No probe heat failure was indicated. FO was hand flying aircraft in IMC at FL260 without AP (Autopilot); AT (Autothrottle) or FD (Flight Director). I decided to [request priority handling] and return to ZZZ1. Received vectors back to airport for ILS XXC. Planned overweight landing and ran that checklist too. While on vectors to Runway XXC; the IAS DISAGREE messages disappeared from both PFDs and airspeed indications appeared normal again. Continued to ZZZ1 and landed uneventfully but overweight at 150;500 lbs. Landed smoothly and let airplane roll down runway; ARFF confirmed no hot brakes and followed us to the gate. I checked with one of our Technical Pilots two days later to see if a cause for the IAS DISAGREE had been determined. I was told that the FO's pitot tube had been replaced and the airplane had been returned to service.We were in IMC and icing conditions at the onset of the event but engine anti-ice had been selected on as soon as we entered IMC conditions. Again; no indication of a TAI or probe heat failure. Even though the malfunction cleared during decent while on vectors for the approach; I elected to stay with the plan of returning to ZZZ1 without delay which I had communicated to ATC; Dispatch; the FAs and PAX (Passengers). This report is informational only. I have no suggestions.