Since a KDE/Plasma 5.x update a long time ago I had the problem that changing „File type options“ (what program is used to open a specific file type) in Dolphin didn’t work anymore.
I could change everything but the moment I closed and opened the „File type options“ properties again it was reset to default.
In my case there were many different entries for the same file type in ~/.config/mimeapps.list. The changed settings were saved properly from within Dolphin but probably not read correctly.
I removed the file ~/.config/mimeapps.list and then changed one entry for file type options in Dolphin to create a new one. This fixed it.
Note that after this you need to set the default applications in „Systemsettings“ -> „Personalization“ -> „Applications“.
Instead of removing the file you may also delete the specific entries you want to change permanently.