I think you misread my original message. I trained stamina during the week of the All-Star break.
So what's the problem with training stamina at the ASB and then losing a visible level at some point later?
Since the stamina gains and drops are slightly randomized, that player probably experienced a relatively low gain and one or more relatively high drops. It happens.
"I don't know half of you half as well as I should like; and I like less than half of you half as well as you deserve."