I'm using continuity-3.0.0-beta.5+1.21, with mods in the second picture installed.
Shader pack: Nostalgia VX
Resourse pack: AVPBR retextured R7-B2
Apparently, right hand should have been a sword, and left hand should be a Totem of Undying.

After doing some tests, I found enabling "Parallax occlusion mapping" will cause this problem.

And Vertex Attribute Fix must be enabled to fix this issue.

Does this happen without shaders?

No. But without continuity, there's no issue too.

Still, report this to Iris as well.

Sure, no problem.

This was just confirmed to be an Iris bug. Use the above linked issue to track this problem.

IrisShaders/Iris#2369