When away from a settlement, the Pip-Boy's workshop section on the data screen may show incorrect values for a settlement's statistics, such as water and beds. This occurs when the player character enters an exterior cell close enough to a settlement such that only a subset of the settlement's exterior cells are loaded into the simulation engine. The result is that only those objects in the loaded cells are included in the statistics. Unfortunately, the settlement will experience the expected negative consequences of this, such as the happiness score dropping, and settlers may become unassigned from their beds. The effect will be more noticeable in larger settlements, such as Sanctuary Hills, which are composed of a larger number of exterior cells, and in the case of Spectacle Island, this can occur without even leaving the island. It will not affect settlements which only consist of a single cell, such as County crossing.[verified]
This is a fundamental limitation of the simulation engine, so there are no methods to permanently prevent this from occurring, though several workarounds and temporary fixes exist:

Returning to the affected settlement and opening the workshop should correct the statistics. In the case of Spectacle Island, you may have to travel to the center of the island to ensure all the cells have loaded.
Replacing all pre-existing beds in a settlement may prevent settlers from becoming unassigned from or not assigning themselves to these beds. On PC, if scrapping is not possible, selecting the bed to be removed in the console and using commands disable and then markfordelete will permanently remove the selected beds.
One can fix this by group selecting preexisting beds and moving them onto a floor. Scrapping or storing the floor will then delete the bugged bed. This feature is disabled after 1.6 patch.
Manually assigning all settlers to a bed using the command option in the workshop menu may prevent settlers from becoming unassigned from their beds.