Welcome to the World of Alzheimer’s disease. There are no specific rules, just general categories. Everyone who goes through it does so differently. One minute coherent, next minute...nothing.
I lived through 3 years of it with my Mom and 7 years with my Dad. Before that, I lived through 5 years with each of my grandparents.
I have always likened it to them being like a corrupt computer hard drive and CPU...nothing it does makes sense or will be the same with the next input.
All you can do is keep them safe and comfortable as much as you can until the end. That’s all you can do.