Give manually setting your ftp a shot
I was fiddling around with w/kg and toggled my ftp up and down(to the tested level) and my levels “fixed” themselves.
I havent done very much structured training due to Ironman last month, therefore AT thought I was losing fitness due to not logging any rides. But I’m still in great shape. I suspect in the background your “assumed FTP” degrades over time. Toggling the FTP number triggered it to say “Hey, your ftp is still actually good, lets pick up where you left off with your progressions levels for the last trainer workout you did.” Manually setting your ftp is the same event as taking a ramp test thru the robot eyes of AT.
All speculation of course.