Custom models UV2 lost after build

I created some editor scripts that basically add a custom planar mapping to the uv2 of a model. This works and stays persistent if i close and reopen unity, play in the editor. But when I generate a build. Then the uv2s values are lost/reset to 0.

The model asset is an FBX file.

Is it not allowed to modify mesh data in a fbx asset? Does a build reload that data from the original fbx?

I tried adding a AssetDatabase.SaveAssets() but it didn't seem to help.

Any thoughts?

Thanks