As others have said…FTP is not the only measure of improvement. Can you hold SS efforts for longer? Is your repeatability improved? Can you ride for longer? Can you do hard efforts longer into a long ride? etc etc.
Also, if you have been struggling with build workouts while only getting 6 hours of sleep then you are probably just over fatigued. Build is really intense and requires much better recovery than the base phase. I’d imagine you’re just over worked/under recovered.