I don't know the logic of the underlying code, but look in the installed d3lphin.desktop file. You'll find two keys: MimeType and InitialPreference. Experiment with commenting out one or both and see how that affects your usage. You might need to restart Trinity or delete your tdesycoca cache files. Probably the best experiment is with a temporary fresh profile. That way no profilerc changes will affect anything.
I don't believe a distro-specific patch should be necessary. We should patch the behavior for everybody. Dolphin should not override anything until the user explicity starts doing so in the file association configurations.
By the way,please share the exact steps you used that I can duplicate that prompted you to start this discussion. :)
I'd like to test this further with other apps too.
Darrell