Longer title: When a patient is ordered to drop a weapon they will no longer be a patient. (Wouldn't fit in subject line.)
When I assign someone to a hospital bed and then click to unequipped a weapon, to equipping someone else, they will drop the weapon and then run around like they had been cured. I have to force them back to a Hospital bed to finish healing. This may be intended behavior but I didn't expect dropping a weapon would cancel any previous orders.
Ver A13 (0.13.1135 rev816)
You interrupted their resting to do something else, I don't see how this is a bug.
When ordered to do something, they will do that task and forget all previous ones. You can change the value for [Patient] in the Work-tab since it is technically a "job" or duty. :)
There is a technical glitch to get around this. If you go into their gear tab and right click on the weapon and tell them to drop it from there, it will not interrupt their current task, even sleeping.
It's by design, for now, since you wiped one order with another.