Just make a fast try re-xporting on one object.
when you export back the asset from the engine to fbx do you see the overlapping problem? if you see and fix it after remporting/rebuild get rid of the issue?If there are still problem try to re-export from ue4 and check if it retained your fix, generated intelligible but ovrlapped uv,generated just trash in uv coords.
This may help out throubleshooting or at least provide more detailed scenario about what’s happeing with your models so we can start from there.