I'm experiencing the same problem as some other people have noticed in Blender 2.82 (win64) - when trying to assign material to a face, the surface suddenly becomes transparent.
Here, in the left mesh, I have the grey material as the default. When I assign either of the material.salmon or material.tuna (both made from default materials with only the base color changed), the faces become transparent.
In the block on the right, the primary material is material.tuna and the missing faces have been assigned material.salmon. I've created a small block to the side using only material.salmon to show that it's not the material itself.
In Vygintas' question Assigning faces to new materials, Martynas Žiemys recommends checking
- blend mode (in my screenshot above, my blend mode is opaque)
- eevee vs cycles (my renderer is set to eevee)
- face orientation (all my faces are blue, facing outward)
In Mins' question How do we assign a second material to selected faces in 2.8? DarkSoul asks if alpha has been enabled. In my case it has not.
Any suggestions would be appreciated.