vip1869 It looks like your setup is working as expected. OneDrive pretends that files are stored locally, while they are not. Any application reading contents of your folder, including Tonfotos, is thinks that files are just there, but once it tries to access them, they will be downloaded first, and will be cached locally for some time after that. However, it is not clear how long they stay cached, as this is something only OneDrive controls.
Tonfotos needs to access every file during first indexing once, since it creates its own thumbnails, reads metadata, recognizes faces, etc. However, later on it does not access those files unless you are actually trying to view them, or view its metadata, or edit metadata, etc. Long story short, once files are indexed, Tonfotos does not need them to be stored locally anymore, even if you are actively working with faces and people. However, once you try to view individual photo, it will be downloaded again, obviously.
Hope that explains how it works.