If you install Orbot from the public F-Droid repo, then the key uses there is the F-Droid signing key, since they build Orbot from source.
If you install Orbot from Google Play, the Guardian Project F-Droid repo, or the direct APK download, it will be signed with the Tor Project signing key.
It is likely you switched from one stream to the other accidentally.
Yes, so there is a problem which is Orbot is in both F-Droid.org and the Guardian Project repo. The F-Droid app makes it hard to know where you are getting it from.
15.1.2 is indeed from us, and from our F-Droid repo.
I agree Orbot should be removed from the main repo.
Also though we are working with them on a new system where they would build the source code, compare it to ours, and then use binary signed with our key, if it matches up.