On a regular basis, and on all servers I play, the weapon disappears from the first hotbar slot. All other slots function normally and without problems.

I'll be playing in a game, and without leaving the game I will look down and notice that a weapon I assigned to the first hotbar slot is no longer there. Testing the slot by pressing "1" proves slot is in fact empty.

I would assume that this could happen when changing servers. But, it happens while playing on a server. BTW, I don't server hop for loot. I only leave a server if I get the "No message" popup, which tells me the server is about to go down.