Alright, I have what I think is a workaround, but it’s another highly domain-specific one:
The assert goes away if I enable Nanite on all of the objects in question. The assert normally hits when I do what I believe is a complex trace (foot IK), and I’m assuming Nanite drastically alters how a complex trace would have to work, hence no assert.
Presently I’m running no-Nanite because these objects also rely on WPO, but as of 5.1, Nanite supports WPO, so, I guess that’s An Workaround, at least? If not a great one. Hopefully it works for others.
EDIT: According to a friend, Nanite physics and complex traces actually use the fallback meshes? So theoretically this workaround would also work in Nanite fallback mode, maybe? Because it functions by pushing the Chaos-angering meshes through the LOD system, which pops out a mesh without the bad data.
By the same token, you could also possibly avoid this assert by never doing complex collisions/traces.