My best guess is that this is due to other plots subtracting from your food. Industrial Plots especially subtract a LOT of food. Similarly, your issue with food going from 10 to 18 sporadically might be related- perhaps an Industrial Plot is getting powered and unpowered erratically, causing the issue. I'd recommend checking your plots for any negative food values and seeing if the math checks out. I've also found it far more efficient to use agricultural plots instead of "regular" farming, as level 3 farms give a LOT of food- and also require less space, and fewer settlers, to manage. You might not be the tryhard minmaxer that I am, though, in which case, farm however you want!
I've had similar issues as well. I'll bet dollars to donuts that it's related to the navmesh issue kinggath highlighted earlier. In my experience it's been mostly cosmetic, but if it's affecting the productivity of your settlements, let us know! Not that I have anything to do with it but I can try to help you!
Also encountered this issue, I've noticed that saving and loading winds up kicking SS's notifications in the pants and I'll sometimes get six or seven messages immediately notifying me of new buildings or updates. Play for a few hours, save, and load your game- if the same thing happens to you, then maybe my solution (however inelegant) will help you as well.