What version of td are you running here?
I hit a bug some versions back where vertex indices for each poly we’re offset by one, creating some weird normal artifacts and affecting uvs as well. Think all that’s been fixed on latest though.
Definitely feel you on the buggy import/export. Fbx is sort of the wild west too.
I’d recommend simplifying your blender export so more and more basic meshes until the problem goes away, then recreate the problem with the simplest geometry. Always helps to identify if the issue is an export or import bug and where exactly.