Operating system: Windows-10-10.0.17763 64 Bits
Graphics card: GeForce GTX 1080/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 419.67
Broken: versions: Blender 2.79 5b8ac9, 2.80 (sub 57), branch: master, commit date: 2019-04-16 06:07, hash: rBedc1b0167518
Short description of error
FBX from Maya imports with incorrect UVs in 2nd channel. UVs that have not been unwrapped in Maya either show as the UVs from the first UV channel (2.79) or are scaled to single pixel at (0, 0) (2.8).
I realise that the Blender FBX implementation does not use the Autodesk SDK, but the FBX SDK has a pUnmapped flag that indicates if the polygon vertex does not have an associated UV and it looks like Blender is not parsing this correctly. It is my understanding that the FBX specification allows individual faces to not have a uv assigned within a uv layer, which is somewhat odd because the uv set itself is received at mesh level, but this alone does not mean all the faces are using it.
For more details on pUnmapped see the following: https://help.autodesk.com/cloudhelp/2018/ENU/FBX-Developer-Help/cpp_ref/class_fbx_mesh.html#a806ac0979b1920ba8c23eebe916d3f5d