SUMMARY
When dolphin is opened over SSH X forwarding, clicking on a file (e.g. abc.png) opens the corresponding graphical application on the DISPLAY of the remote machine (DISPLAY=:0) and not on of the SSH client.

STEPS TO REPRODUCE
1. ssh -Y remote
2. dolphin .
3. click on a .png file

OBSERVED RESULT
the gwenview window opens on the remote machine.

EXPECTED RESULT
the gwenview window should open on the client


SOFTWARE/OS VERSIONS
Windows: 
MacOS: 
Linux/KDE Plasma: Manjaro
(available in About System)
KDE Plasma Version: 5.14.5
KDE Frameworks Version: 5.54.0
Qt Version: 5.12.0

ADDITIONAL INFORMATION
Most likely, it affects all versions of plasma. 
Instead of directly opening the clicked file, it seems dolphin sends a message to plasmashell to open the file; dolphin is not the parent of the new graphical process (e.g. gwenview, the parent is plasmashell.
plasmashell uses DISPLAY=:0. so the new window opens on the remote machine.
https://unix.stackexchange.com/questions/496814/ssh-x-forwarding-gui-window-opens-on-remote?noredirect=1