It's a bug, but it only ever happened on my old laptop, rejoining the server seemed to stop it. I think there was also something you could type into the console that fixed it, it was either "record demo" or something like "resetsounds". I'll make a note of checking, because I remember one of them working.
If you record a short demo it usually fixed any kind of visual glitch. I've bound a command to record a demo and stop it directly, which seems to fix it.
ah yeah, then I think it's
reset sound in the console that fixes it then, I'm not sure if that's the exact command, but try a few variants because there's a sort of autocomplete feature on the console and you'll know it when you see it, if you're not sure then leave it, who knows what some of those comms do.