This is a vanilla disparity in which the arm swinging animation differs depending on what the player's main hand is currently set to. I may have accidentally found side effects of this when I reported #71, although that issue has been marked as fixed since yet this issue remains.

I'd recommend reading the Mojira bug report for more info and reproduction steps, but the attached screenshots should give a basic idea as to what this issue is responsible for.

Hey there, there's a mod called Debugify which fixes bugs reported in the MC bug tracker. You can make an issue there and it will be fixed probably!

Yea, I'll make the decision not to fix this. There are mods like the mentioned Debugify that fit this more.