I was using the tavern to upgrade Judge from 29 to 30. I selected 3 1 star, and 1 2 star which showed the xp meter as full/maxed out - "max level". Clicked button, took the silver, showed him hit 30. I then went to chamion screen and it still showed him as 29. I went back to champion screen and the overview showed him as 30, but his details still said 29 and required another 1 star and 2 star to max.out again.

I should also mention that I have another level 30 that remained maxed out. Both were also ascended 1 star prior to all of this (if this helps at all).