Yes, failure to thrive is a favorite, but a real diagnosis, especially in those with dementia. However, in preparation for demise, the body senses that it does not need food. Thus, appetite decline may begin anywhere from 120 to 90 days. Fluid intake will remain stable until a few days prior to death.
I am not implying that your loved one may be in the dying process, however, it may be something to consider. I just wanted to clear up and perhaps possibly have explanation for some of your concerns.
May God bless you and give you comfort during this time.