with gas mask on there appears a horizontal black stripe and damage indicators (blurred vision when PC is hit) are messed up and make the whole screen go very blurry (so you can’t see anything through it). When I take the mask off the stripe disappears but the damage effects still happen. Don’t know how could you reproduce it. It happens both in SP and MP. I’m playing on PC (i7 7700 2,8 and GeForce GTX 1050).
Perhaps your mask has a crack in it, and someone used scotch tape to patch it. Lol!
Try a new one - literally! Is it all gas masks? Try a different color one if you have found any. Grey, green, black. Can’t really see it being the issue of only one of the color ones, but you never know, right.
I too am experiencing a similar issue following this latest update. When I get hit, 3/4 of my hud goes very dim. Can’t see much. Not wearing any gas mask or any kind of glasses at all. The game is unplayable now. I’m also playing the PC version (i7-7700K, GeForce GTX 1060, Win 10 1909)