I, as well as many others, have experienced this issue for a long time now.

I do not know when or why it occurs.

One of my weapons becomes invisible. Let's say I am using S/B and Resto. My S/B will become invisible after a bit. It might be a bug related to swapping to fast or something; I don't know. But it's very annoying, and it has been around for a while.

To fix it, I have to equip/re-equip armor. I usually equip/re-equip a disguise.