Operating system: Windows-10-10.0.18362 64 Bits
Graphics card: GeForce GTX 980 Ti/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 388.13
Broken: version: 2.80 (sub 75), branch: master, commit date: 2019-07-29 14:47, hash: rBf6cb5f54494e
Short description of error
The Face Orientation viewport overlay does not work on an object which has a material applied that uses an alpha value lower than 1 in its Viewport Display settings. If the object has multiple materials applied the overlay will only work on faces that have a material applied which doesn't use transparency in the viewport.