Hey

so a couple of things.....

Is the model in a group? Does it have transforms or even model history, these are basic areas to check, so freeze transforms and delete history, and if you do have a group node, take the mesh out of it for the time being and delete it, you can always add a fresh one later.

Are you using the Maya standard chequered shader? If you are, get rid of it and opt for a numbered uv colored chequer map. Your life will be a bit easier when adjusting if need be. As well, up the uv coord repeats for the map so you get a better level of detail so you can determine stretching. Usually around 10 to 15 is good as a starter.

Depending also on the renderer you are using, you are going to get issues, so if you are using legacy viewport or viewport 2 you need to pick one, Maya is notoriously buggy picking between the two when it comes to mapping surface and if you switch between them it just gets confused. But if you are using 2020, then you will only have viewport2. So that will be that solved, otherwise pick one from the other two if you access to either.

Mesh exports. Why are you using an old format such as FBX to attempt a clean up? Its not really the most reliable source in the world. Export either an OBJ or an MA file, the MA file will be slightly bigger, but will be clean.

Then hopefully reproject your uvs on the Y axis and you can start to tweak....good luck

Jay