This issue was fixed completely (also for existing games) in Patch 1.52.100 for PC/Mac and in Patch 1.16 for console.
My vampire has reached level grandmaster and now I don’t get any extra power points anymore. The bar is full but doesn’t move anymore.
According to SDPhantomGamer there’s a small error that only allows to level for a single time instead of multiple times. Because of this, you can only get extra points once and after that, the bar gets stuck.
Make sure the faulty Vampire is active, then open the cheat console with ctl-shift-c and : stats.set_stat rankedStatistic_Occult_VampireXP 1592.
This should temporarily fix them but the issue will unfortunately reoccur.