Thank you so much for your feedback!
Basically, at the moment there are two ways to generate food:
1. windmills
2. fruit trees
Fruits, created by fruit trees, decay the next morning (or grow new trees!). and windmills on grassland tiles produce flour, that does not spoil. So this big drops are probably because of fruit trees being the main source of food.
Damn, 49 is so close! тт_тт Sadly, you are not alone - my playtesters reported this problem as well, so I am currently working on the food system that would be more transparent for the player - new update is planned in a week or so!
I also added the issues you discovered to fix in the next update.
Again, thank you sm!