Modpack version
1.82a
Issue
Noticed my beheading III laser gun was not giving any wither skulls (1 in 40 kills) when I used it in offhand with food in main hand. After moving it to main hand, I immediately started receiving a skull every other kill. Not sure if other tinkers modifiers such as looting are similarly affected.

I'm afraid there's not much we can do about that, 1.12.2 mod support isn't really a thing anymore 😄 thanks for reporting anyhow.