D,
While I can see an elite and experienced athlete temporarily plateauing their FTP on an 80/20 plan, reverting is not expected.
There are many reasons why an athlete, on an 80/20 plan or otherwise, would revert. These are the questions I would ask:
– To answer your direct question, supplemental strength training may be required for many athletes to improve. What was the strength training plan for this athlete?
– Was power to weight ratio going up even if FTP was going down? Then raw FTP doesn’t matter.
– When analyzed, at what intensity balance ratio did the athlete improve FTP? If not at 80/20, did improvement occur at 70/30, 90/10?
– How long was the athlete on the 80/20 plan? It really needs a dedicated 12 weeks.
– Did FTP go down by 1 watt or 30?
– Was this bike and/or run FTP?
– How was the adherence to the 80/20 plan? I’ll sometimes pick up an athlete who believes they are following 80/20 but upon inspection the training is not fully aligned with the 80/20 principles.
– Would the athlete be willing to allow me to analyze their training?
David