Quarterstaff Bell vs. Act 4 Behemoth

When using the quarterstaff bell, the behemoths are immune or cannot be hit by all damage. They regain the ability to take damage after the bell expires or hits are used. This occurs on white mobs and rare type enemies. I have not seen this issue on other campaign mobs and have been using bell as soon as i was able to cut the gem.
Thank you guys for all you do! Happy Holiday!
*edit* Just wanted to pop in as I've hit endgame now. All behemoth in endgame content are immune to bell damage as well. This does not appear to be a campaign specific issue. Happy Holidays everyone!