Ok, so, as expected, the explanation is still the one above, you can’t upgrade from different stores.
Now, what do you actually see?
- Pixel 2 - app from F-Droid, can’t update from Google Play…as expected.
- Samsung S7 - app from Google Play, can’t update from F-Droid (no, it’s not about version number), the issue is that you’ve forgot (I guess) exactly where you’ve installed this app from.
To prove my point, on the Samsung S7, go to F-Droid settings, I bet “include incompatible apps” is turned ON, right?
That’s why you see both there, on the S7, now, toggle that OFF and surprise… F-Droid versions disappear… because they are… umm… incompatible… different signatures.
FYI, Tutanota stores all the e-mail on the server, you can simply uninstall your app from Google, reinstall latest (yes older at the moment) from F-Droid and carry on.