A bunch of things in there. First, what trainer and power source are you using for this? Any change in that setup?
Second, the second phase of SS base is much harder than the first, with sweet-spot turning into threshold, intervals stretching, and VO2Max being added. It is far from uncommon for people to scrape by phase 1 and fail in phase 2.
Third, that 320W test followed by a 250W test sounds suspect.
Fourth, I don’t understand the “switching back to Peloton” (gawd it grates me how people will write that word) part.