KDE & Plasma users

I'd like to change the preferred-application order for some mimetypes/file extensions, and trying to do that through the "File associations" tab of the System Settings Module. But the changes don't stick.

A look at .xsession-errors shows that some files must be corrupted. For example I see (NB: this isn't the mimetype I'm trying to change):

Mimetype Comment Dirty: old= "application/vnd.amazon.mobi8-ebook" m_comment= "Amazon KF8 ebook format"
Entry  "application/x-mobi8-ebook"  is dirty. Saving.

and similarly for several other obscure mime-types (e.g. "x-x509-ca-cert").

I've tried a reset as described here, by renaming the ~/.local/share/mime directory, creating a new ~/.local/share/mime/packages/ one, and calling update-mime-database ~/.local/share/mime. But it doesn't help. I manage to do and save one change in the File Associations settings, but after that, other changes won't stick anymore.

Something seems corrupted upstream.

Any suggestions? Cheers!