Assigning materials in Edit mode causes faces to disappear in this simple example. In more complex files it distorts mesh even more.
System Information
Operating system: Windows-7-6.1.7601-SP1 64 Bits
Graphics card: GeForce 310M/PCIe/SSE2 NVIDIA Corporation 3.3.0
Blender Version
Broken: version: 2.83.4, branch: master, commit date: 2020-08-05 06:00, hash: rBc113af82881a
Worked: (newest version of Blender that worked as expected)
Short description of error
Assigning materials in Edit mode causes mesh deformities
Exact steps for others to reproduce the error
create mesh
assign material
create 2nd material
select a face in edit mode
assign 2nd material
This occurred in all iterations of 2.83