We are getting ready to migrate from SP 2010 to SP online and I noticed something strange - if I query a user who hasn't signed in yet to our new SharePoint site using _api/web/siteusers(@v)?@v='User i:0#.f|membership|someUser@amc.edu' I get no results. If I then just go to permissions in SP online and check if someUser has permissions (which they may or may not have), then by merely checking someUser's permissions now _api/web/siteusers behaves correctly. Does that make sense?
Note: We are using AD
Another strange thing is that the people picker in the permissions form instantly recognizes someUser so it seems half of SP is aware and another half is clueless