I think it is time to replace the on-road power meter. It's the shorter crank I use for TT and road bike. It reads from 15 to 30w less than my Kickr and another meter on another bike (longer crank). Since it reads less than a smart trainer, that means drivetrain loss makes the difference even more.
When your ftp is under 300w, a 30w difference is massive.
By the power readings of that thing, I must be breaking physics on my TT rides to go the speeds I do on such low power. Must have a world record CdA even without ever tunnel testing. LOL.
Starting to not be something I can routinely ignore.