I have created a bug report concerning Gimp. But it was no Gimp issue. Finally I created bug report https://gitlab.gnome.org/GNOME/gimp/issues/3156.
From there I got the explanation:
“This is a byproduct of the confinement that the snapd enforces on strictly confined snaps. /tmp is unfortunately unique per snap and so the GIMP snap cannot access the path from the host system.”
Ok, this may explain why it is not a bug of Gimp or Gnome and that it is a consequence of a design decision in Snap. But the result is quite questionable and makes average users wonder about weird computer behavior.
I would recommend that you think about circumventing this problem by some way of making the directory /tmp accessible to programs installed by snap, because e.g. screenshots and other stuff might be placed there and ghe user may want to process it by a snap-application.