Are you sure this is a bug? And that Nordeus haven't changed things without telling us?
I have the same issue with my 2nd team. Started (perhaps) 3 seasons ago. Tanking team with too big a stadium. Attendance maxed out for its level but no possession bonus. Last season promoted and now this season I have a bigger attendance and 2% possession bonus.
I can't get any more fans into the stadium until I level up again (and again?). So I can't get the full 5% bonus?
I'm wondering if this was a half arsed anti tanking attempt and that teams like yours and mine are being "penalised" for having overbuilt our stadiums and then tanked at lower levels.
What do you think?