Sorry, I haven't really had any time to look into it much yet because of Medical Bullshit, but after some thinking (which is unfortunately all I can do right now) and asking around, you're basically right?
So the way UVs work in the older engines is that they're contiguous, which I sort of keep... but which leads to problems in Unreal, because it Does Not Like That.
The solution is apparently to go into the static meshes that have problems and check "use full precision UVs"?
(which is and absolute pain right now because there's no bulk way of doing this, but if that solves it for you, I can see about making this the default)