It was just a visual bug, and it seems like I was the only one having this issue during the run.
One of my friends was completely missing his weapons from my view, and his apparel reset back to the default. Everything looked completely normal on his screen though. It looked quite funny on my end because he would just charge into the room empty handed and the sleepers would suddenly just drop dead.
There were no bullet tracers when he was shooting, either.
I don't know the exact cause, but I suspect it might have something to do with the exact timing of when I joined the lobby. Maybe joining at a weird time messed up the sync? It completely fixed itself as soon as the expedition ended, though.
It doesn't affect the game much, so I don't think it even needs fixing. I'm really just sharing this because it just looks funny