I noticed already that only world-spawned weapons will disappear.
By world spawn, I mean weapons manually placed in the world, on display stands ( 1h swords in the fort cellar by the port, Esmeralda's shop ), on the ground ( the bow just after the first orc fight on the beach, before you enter the town ; the bow in a field with a scarecrow at the middle, where you encounter a bomber and a few skeleton archers ), or the 2handed sword in the skeleton ribcage in the cellar of the lighthouse.
Every weapon crafted, bought, looted, or found in containers will be perfectly OK.
We 'finished' the beta with a friend as the host, and I kept the starter bow almost all the way because of this. He never got a single weapon from the world, so he never had the issue.
AFAIK, it only concerns weapons, and I'm not sure about armors and shields. I fear they may suffer the same fate, though.
In short : if you want to keep your weapon for the alpha duration, try to buy them, or seriously hold onto any looted one :p