Have you compared your power on the trainer with the power meter in the same workout? For me the power pedals are consistently 13 w lower than the turbo.
I did a full base phase and a full build phase on TrainerRoad since I joined in October. I only failed 1 workout (Warren) and my FTP dropped from 280 to 270, then 260 and is 255 currently.
So I feel you, welcome to the club.
I sometimes run them parallel, such as today. Results:
Tacx Flux (trainer)
Avg. power 249
Favero powermeter (pedal)
Avg. power 245
Similar to you a difference of -12W from the pedals.
Additional info: I did 9 workouts outside and 9 inside since I have the pedals. The ones inside went just fine, no early dropouts, no lowering of power settings. For instance Spruce Knob 2x30 min. SS without problems.