Cool, well I have another player that got to prominent stamina this week, so I will see if he drops down to proficient next week.
If he doesn't drop down to proficient that means that stamina doesn't drop for all players each week and just drops randomly. On the other hand, if he does drop, it probably means that every player drops a little bit in stamina every week.
Also interesting that stamina training occurs before the stamina drop, so that even though I trained stamina this week there was still a drop in stamina.