My 87 yo mom with dementia has been on the roller coaster ride in terms of eating. When she first moved in with me 18 months ago, she had great appetite and ate normally and regularly. Slowly her appetite declined but then she went on an extremely irregular eating pattern. She would go without food for half a day, a day, two days and her record is about 50 hours of no food and little water. But then she would start to eat again. Currently she is on the high side for food. Last night at 1 am she emptied one bag of chocolates, two cookies along with other snacks after three good meals during the day. I’m so baffled.
Another piece to the puzzle could be Mom's teeth were hurting due to something being stuck between the teeth, so she didn't eat but now everything is ok.
Another possibility is the type of toothpaste she is using. Those whitening toothpaste or one that contain mouthwash can mess with ones taste. I found for myself that buying a very mild toothpaste made a world of difference for both my sig other and myself [we are 73]. We changed over to organic toothpaste.
And let's not forget, dementia can be quite complex.