How to reproduce:
1. Open the attached "FBXExporterQuadIssue.zip" project
2. In the Project tab, select the "IN-126856_TomcatWhiskers ascii.fbx" file
3. Export the file to FBX (GameObject -> Export to FBX, then rename the new file and press Export)
4. Select the exported FBX in the Project tab
5. In the import settings for the newly exported FBX file, set "Optimize Mesh" to "Nothing", enable "Keep Quads" and disable "Weld Vertices"
6. Press the arrow next to both FBX files to expand them
7. Select the mesh of the exported FBX
8. Observe the "Indices" field
Actual result: There are 2 submeshes, both quads and triangles
Expected result: There is only 1 submesh (quads), as in the original file's mesh
Reproducible with: 3.0.0-preview.2 (2021.1.0a2), 5.1.5 (6000.6.0a2)
Reproducible on: Windows 10 (By user), Windows 11
Not reproducible on: No other environments tested