I have PC, several settlements and don't experience most of the glitches found in many threads; ( version with patch beta 3.47 I think); however suddenly all of Sanctuary's farmers became spontaneously unassigned. I reassigned them after I left and noticed on the map summary food (production) was listed as zero. The only unusual thing just before is that I rang the town bell several times to find the new settler (and assigned him to the farm when he finally showed up). I have two bells. One in the center of town and one near the farm. The defenders assignments were undisturbed.

Do you have a large population? I've had this happened in Sanctuary Hills and Spectacle Island, each with 30+ settlers. I've had beds mysteriously becoming unassigned also in a smaller settlement (https://www.youtube.com/watch?v=8kONA3uCfd8), but that hasn't happened often. Basically, for a large population, the game may take a LONG time to assign something. And if you interrupt the process, you get glitches. After you assigned a settler to a bed, crop, or store, STAY IN WORKSHOP MODE until you see the message "this resource is now assigned", which may take 30 seconds to a minute to show up. Also, DO NOT ASSIGN THAT SETTLER AGAIN. Only assign him or her ONCE. Don't redo the assignment. Redoing the assignment while the game is processing that very assignment will screw things up. But it's okay to assign OTHER settlers to other tasks while the game is processing the previous assignment.

If your resource shows zero, it may go back up to the previous correct value by itself, perhaps very slowly. Your beds may be at "zero" too, but you don't see it. That's because the workshop mode only shows the number of beds BUILT, not the number of beds ASSIGNED, whereas with crops, you actually see the number of assigned crops. That's why you may have unassigned beds but don't know it. Sometimes, unassigned beds never become assigned again, even though beds are supposed to be auto-assigned. To fix that, I have to store the bed, build the bed again, and it will be auto-assigned, perhaps after a longer-than-usual wait. This is a headache in a large population because you have look at every single bed to see if they are assigned or not.