I have a friend whose little 13 year old Jack Russell (also in splendid physical health) is doing exactly the same. My friend is happy to continue feeding her by hand, and to tempt her by adding bits of cooked meat and tuna etc to her dish. The little dog eats a bit, then walks away, and sometimes comes back for it later, especially when interacting with someone.
She has also had health checks and blood work done and she is healthy. The only thing that may be happening is she is at the moment grieving for her doggy-sister and best friend who died in November. So this seems to be mostly emotional.
My friend's dog doesn't have dementia though.
It may be that you will have to do a similar thing with your girl. Tempt her, interact with her, add tasty bits of meat or fish to her food, or find some healthy additive-free dog treat bites to break up and sprinkle on top of her food?
If you have to hand feed her, so be it. There is something so nice and intimate about doing that. 10 minutes or so, just your dog and you, and tasty food, and the whole world goes away.
We all change as we grow older. Sometimes the changes are inconvenient or confounding, but if we can work with those changes, with Love, life is sweeter I think.
Cook her up some plain boiled human-grade meat cuts, and add that to her food.