Here’s a rule of thumb for sweet spot: you should be able to ride at sweet spot for roughly two to four times as long as you can ride threshold (SST TTE = 2~4x FTP TTE). People that do a lot of long rides will be up near that 4xTTE, if you’re training mostly on shorter rides it will be close to 2x. And at the same time, your FTP TTE needs to be a minimum of 35 minutes (which you can hold as one 35 minute interval).
If you can’t do at least 2x your FTP TTE when riding at 90% FTP, then your FTP is set too high. Given that FTP TTE should be 35 minutes at a minimum, the fact that after weeks of training SST you only got to 50 minutes of SST is all the proof that I need to confidently say your FTP was set too high at that time.
I highly suggest that you only use Kolie Moore's FTP test protocol for setting your FTP, at least until you are very confident in feeling where your threshold is. And don’t bother with raising FTP by less than 10W.
Finally, FTP does go down temporarily with fatigue, poor sleep, poor fueling, and high life stress. Make sure you are taking all the necessary steps to reduce those factors.