Is a bug on tracker related to the new photos app. Quite nasty, since in shares photos won't open at all.
It was supposed to be fixed with 19, but its still there. I wait day by day for a new release.
Seems like the behavirour is still the same.
I found this post: https://help.nextcloud.com/t/e…-to-your-photos-app/86921
I suggests to install the old gallery app to make photos from external storage accessible with the photos app.
Guess that is what i am heading to then.