Search API with Application Permissions returns no results, Delegated permissions do

Maybe good to know: I have a list, on which I try to find listitems. The list returns 0 results, searching using multiple options (no ManagedProperty, on RefinableString, ...) when using Application Permissions. Except for 1 listitem: this one I do find, with both permissions (application, delegated). I accidentally found out.
I tried to figure out what is different on this listitem, but couldn't find anything.

Edit: the only difference I see, is that the Author of that one listitem is an actual user:
| upn | display_name | some_large_guid i:0#.f|membership|upn
The other listitem's Author's are an app:
| App name | some_large_guid i:0i.t|some guid|UPN_of_app

Edit 2: I created a new listitem myself (not by the app above), and that one I can find as well, by any authentication.