If I changed my training this week, I would understand what you said. And I would like to add a point because I have others players that can hold stamina at prominent level for more than 1 week (I cannot say for real, but I think they can hang 4/5 weeks or more if we don't exceed too much the minimum trainnig minutes)
As this game is a management simulator, in my opinion if we talk about logic, every time you train, the trained players should increase something (an undefined value), a few times the player pop up, but in most of the time the player don't pop up, however if he doesn't popup he should increase something. If we reach the stamina top level and train stamina, it's not logic to pop down, the logic says that, at least, the attribute should remain unchanged.
I also must say that what happened it's not a big deal, I'm not mad with the game, but my point here is trying to report that as a management simulating game, this specific training episode kinda prove that maybe some logic is broken here. And I'm not sure if something like this could happen when you are training other attributes where you are at top level. :o