Are you sure it was purged well enough? These things have a second heater core for the rear hvac. There is some big song and dance of reving the engine with the heat on etc to get all the air out.
The upgraded part is a rt1258, ford has some sort of convoluted part number that is goes by as well. I do think the newer one is plastic instead of brass.
The other suggestions to look at are excellent ideas also.
The rt1258 solved my hot running 2019 that had been hot since new and it wasn’t apparent. I don’t think it ever worked right and it was starting to spike temps ever now and then around 30k miles. I went from 205 trans temp when driving in town and 212-215 towing to 180-190 all the time and maybe 192-195 towing.
Can you tell me what your coolant temps were? I have been looking for a concise reference for this and I think you have the data.
Original Thermostat RT1252:
Coolant Temperature:
Transmission Temperatrue Normal:
Transmission Temperature Towing:
Revised RT1250 Thermostat:
Coolant Temperature:
Transmission Temperatrue Normal:
Transmission Temperature Towing: