I assume this is a bug, but could be something else going on.
I noticed that sometimes when I enter combat as a Werewolf I almost immediately get taken out of WW form. It also happens if I change to WW while already in combat.
It might be related to Ultimate generation but it happens so quickly there is no chance to do anything to generate ultimate, which is why it seems like a bug.
I also noticed in a longer fight when Ultimate was getting low, I got taken out of WW form with 4400% showing on the ULT icon (50k% is max). If the % is right, there should have been quite a bit of time left in WW form.