You should use GLTF/GLB format when working on payday models since it has support for all the features and is the most tested. Your issue might be from a culling box being displayed over the actual mask though I don't think I've seen masks with culling boxes before. It's hard to say without seeing how it looks on your end. What mask did you import? And maybe you can attach a screenshot of how it looks in blender for you?
Hi, i'm trying to import weapons models, not masks. idk how to attach screnshots, i don't see the option here, but here some links:
And i already try that of export them to gltf/glb, and the result is pretty much the same, the only difference is that the rectangle is small.