The calorie level drops sharply for no reason

Don't know if you can check, but something similar to this happened to me because my colonists were cut off from getting to where my food was stored because of a cave in, so the kcal counter only showed the food that was available to them in the area they were in.

I agree with @asmallrabbit. Sounds like the scenario that they pointed out. Your dupes probably were momentarily out of reach of the food, which is why the counter dropped.