Right, so answering my own post here:
It boils down to a puzzling thing about the way IC8/CC4 is solving the movement of the jaw root bone on many of these actor avatars.
Can't see any way to configure this while exporting from CC4.
The moving of the jaw root bone is not caused by the animation/motion itself (as it should be for a correct glb export), but the vertex animation (animated shape-key/morphs) is animating the jaw bone. Inverted, so to speak. This is the opposite of what is expected.
While this seems perfectly legit in an isolated case, e.g. when creating a sequence in Blender, but this method will break down, at some point, in the pipe-line. In this case, when exporting from Blender to .glb.
A solution to this is cumbersome if done manually in Blender, especially if there are many lip-syncs involved, so this, I think, should be delt with in IC8/CC4 prior to export to Blender. The lip-sync motions themselves should transform the jaw bone and not the blend shapes.
Maybe @victorsoupday could provide some more insight?
My 2 cents.