This has happened to me a couple of times but hard one to catch as I’d have to grab a snapshot before and after and I can’t predict when it happens.
Scenario this morning was I had a VO2 ride 2.9 which was labelled Achievable (my current VO2 Progression level was exactly 2.9 this morning). Completed the ride at 103% (not sure if that has to do with it), and post ride, my VO2 level said +0.8 and is at 2.9, which means my progression level was at 2.1 prior to this ride but it clearly said 2.9 before this ride.
Your PL’s regress on their own over time. I believe it’s 2 weeks for most zones. If you hadn’t done a workout as hard or harder than the one you did today in two weeks, there is a good chance it regressed while you were doing it. Happens to me occasionally. It’s not too unlikely considering we’re doing these workouts at the same day and time as we often do and touching each “zone” is not always part of the plan.
This was my first VO2 workout in probably 6 weeks, but thought it was weird 1 hour prior my progression level was 2.9 and when I finished it was still 2.9 but +0.8 on the workout but I could see where you’re coming from with that for sure.