In Spanish, "ana" is pronounced "ON-ah." If I know a family is familiar with Spanish phonetics, or the name appears to be Spanish, I would say ON-ah when i see Ana or -ana.
However, it's used pretty frequently in English names interchangeably with Anna, and I know that. So if I see an English name in an English family, I tend to assume that -ana is the same as -anna. For example, Anabel, Liliana, etc.
Bottom line: I have a guess in my mind before I try to pronounce the name, and if I'm right, great. If I'm wrong, then I correct it and move on. But I recognize that people can pronounce them differently.
I would definitely pronounce Kianna as key-ANNA, while Kiana could potentially be key-ON-ah, or it could be key-ANNA as well. I would have to ask the person to be sure of the right pronunciation.