I had the same thing happen to me twice in the past couple of weeks. “Same” in that progression level dropped and it didn’t make any sense to me. I had a custom training plan built that included 3 scheduled workouts per week. In one of those weeks I felt good on an off day and wanted to add an easy z1/z2 workout. I can’t remember which one it was, but it was easy. I was very surprised to see one of my progression levels drop after that. Kinda made no sense. I mean, if I hadn’t added that extra easy, recovery ride, my progression level would have remained higher? Huh? How is that supposed to be informative?
The second time was when I replaced one of my scheduled, 1 hr sweet spot workouts with a shorter 45 min version due to time constraints. The intensity was actually a little higher, but overall tss was lower due to 15 min less riding. It just felt weird that my progression level dropped because I completed a slightly less difficult workout. The point being that I didn’t become any less adapted or fit or capable because I did that workout. It just made the progression level kind of meaningless to me. I mean, what is that number telling me in that case?