When reading your thread, it seems like you have had great progress, except for some setbacks recently.
Your FTP, be it 250 or 280 is great. Recognize the achievement. Few people get there.
- Gains are harder the fitter you become, trust the process!
- All gains doesn’t equal improved FTP, e.g. improved capability to sustain a certain effort over longer durations. Looking at SSB, I would think that one key expected output is ability to hold power close to FTP for longer durations, rather than the intense effort of FTP-testing. Obviously, there should be certain correlation.
- Being / feeling stuck for a couple of months might be coincidental. Testing is not easy to control. Your previous tests might have been at particular good days, well rested, caffeine, right time of day, best music, great mental attitude, low stress, great cooling, muscles flooded with nitrate… and recent tests at somewhat poor or normal days. What difference will it make if you go train at 250 instead of 280? You might miss some gains, but you might also avoid burning yourself out. I think the potential reward is not at all in proportion to the risk. You might lose some small gains until next time if you are better than current estimated FTP, but if things feel too easy, just increase intensity.
- Consider alternative evaluations: how does my ride feel? Is it easier now? Can I sustain bigger efforts?
Looking at your fitness-fatigue chart above, not surprised you felt flat. You have been increasing your load. Not so strange that RPE goes up if you are detrained.p, either.