Assigning a new material to a face makes it transparent 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.
I have tried a brand new project, with a default cube, and as soon as I assign a new material to a face it becomes transparent as well.

Any suggestions would be appreciated.
I hope this helps others who run across this. This is a known issue when using multiple materials with a number of older GeForce and Quadro graphics cards.


  Blender Known Issue: Drawing error with multi-material object. Faces disappear or showing artifacts with 2 or more material slots applied to same object. When trying to use a second material or more on an object, the faces where it is applied become transparent or in some cases they are distorted as black rectangles that stretch to the origin (this second occurs above all with more complex models than a cube). It happens in Blender 2.81 as well as in Blender 2.82, but in Blender 2.80 there is no such error.


While the bug report lists multiple cards such as the ones below, the list is not exhaustive (my card for instance, GeForce 9800 GTX+, is not listed), the core issue is that the GPU has to be able to run NVIDIA drivers greater than v400. The highest driver that I can install is the GeForce v342.01 driver.


GeForce 210 
GeForce 410M 
GeForce GT 220 
GeForce GTX 275 
Quadro FX 3700 
Quadro FX 3800 
Quadro FX 4800


I can also confirm that as listed in the bug report, Blender 2.80 does work for me.