System Information
Operating system: Windows-7-6.1.7601-SP1 64 Bits
Graphics card: GeForce GTX 970/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 426.00

Blender Version
Broken: version: 2.81 (sub 12), branch: master, commit date: 2019-10-02 16:56, hash: rBd1a1302b01f4
Worked: (optional)

Short description of error
Assigning a material to selected doesnt work well

Exact steps for others to reproduce the error
In that wheel (edited cylinder) assign the selected faces to a material destroys the geometry
of the mesh. a lot of faces become invisible. The normals are ok.
Then an other assignation doesn't work any more.
[Based on the attached assign-bug.blend file]