Whenever I leave Sanctuary Hills, PipBoy shows the settlement's water, power, defense, and number of beds as zero. The happiness rating of Sanctuary Hills begins dropping accordingly. Whenever I return, PipBoy and Workbench mode show the correct values for these quantities and happiness begins slowly increasing. This makes it next to impossible to attract new settlers to Sanctuary, or even retain the settlers I have. Fix?

Do you have settlers assigned to each thing ?

The exact same thing happens to me. And it's not just Sanctuary, every settlements so far where I've manually set up a bed or defences have this glitch.

I've made a thread couple of days ago with no replies.

There's a temporary fix to this that I've found:

While in Sancuary, walk to a next location (like Red Rocket) and use fast travel there. Looking up the pip boy while in your new location, the stats should stay the same now.

I came back to post again because I found the same workaround tonight. You don't have to walk all the way to Red Rocket, by the way. As long as you walk outside the boundary of Sanctuary Hills before you fast travel, it works fine. I just cross the bridge and fast travel from the other side.

Assigned to each bed? Assigned to each water? Assigned to each power? Read my post. The things that drop to zero aren't even things that require settler assignments (other than security, but I mostly use auto-turrets anyway). Like I said, the numbers will change back from zero to their correct values as soon as I set foot in Sanctuary Hills again. But if I fast travel while in Sanctuary Hills, the values for each of those stats (beds, water, power, and security) drop to zero until I go back.

It's not as big of a deal now that I know an easy workaround, but I'd imagine it's confusing and frustrating a lot of players who aren't noticing that those values are changing when they leave. They'll have elaborate, well-constructed settlements at Sanctuary Hills and wonder why their happiness keeps dropping no matter what they do. The mechanics in Bethesda games are already so opaque and vaguely explained, it's infuriating when a difficult-to-recognize bug makes the mechanics even more confusing. If this bug doesn't get addressed in the next patch, Bethesda should at least be making efforts to explain the bug (and its workaround) to players in some public forum.

Maybe this was happening to me today. Got Sanctuary to 80% happiness and then after my play session it was down to like 65%. That's after planting more food and adding more beds.