My dad's symptoms match stage 6 dementia. His symptoms are the opposite of your nan's: he thinks he hasn't eaten when he already has and he'll get angry and start demanding food. That's on some days. Other days he is terrified of eating and refuses completely.
Several months ago he started to have food issues. It's now reached the point where he often gets distressed at mealtimes and doesn't want to eat. Mum has to mash everything and feed him with a spoon like a baby. He can't identify what he's eating and although mum always gives him his favourite foods he always complains it tastes terrible. I've talked to him about it and he can't really put it into words, but it seems like food has lost its taste and he just feels like he's eating cardboard. Plus he has no appetite or desire to eat. He also has a physical problem with swallowing which is another stage, so I guess there might be a real fear of choking in his mind when he tries to eat.
This is just how his brother was before he passed away 2 years ago. He lost interest in food and stopped eating altogether. They had to feed him by tube.
LS