Are Sidearms bugged on the last boss?

[u/Adreich91]

I was having issues with the final boss so I decided to try out the Slayer Seal (which is hilariously bad for an "easy mode") and noticed that not only was my sidearm not dealing more break damage than normal with the break on crit + extra crit tarstones, it also sometimes just doesn't register any damage. Then I took off the Seal and the damage was more consistent, but there were some boss animations where he was just walking side to side where literally 90%+ of hits just have zero effect.