Krita is not using XDG file picker. It’s instead using a generic qt file picker (i think…?). KDE file picker is just more pleasant to use. I’m using Krita version - 6.0.2.1, native Gentoo package.
This always was like that on Linux, if I remember correctly. Also Krita 6 is still beta. I can’t remember Krita ever opening a different file dialogue or maybe my memory is just wrong.
In all versions of krita appimages that I’ve seen, in Settings → Configure Krita → General → Miscellaneous:
The ‘Enable native file dialogs’ value is forced to disabled and is greyed out.
For 5.2.3.1, 5.3.3 (Plus) and 6.0.3 (old Next) appimages, the DontUseNativeFileDialog= is present in kritarc and setting it to ‘false’ has no effect on the file chooser dialog but that may be a limitation of my Linux Mint with MATE installation.
The latest Plus and Next appimage builds do not have DontUseNativeFileDialog available in kritarc.
It seems that the Gentoo repository has an old version of krita Plus in it. (The build tree numbering is confusing, especially with the qt5/6 difference, so I may have got that wrong.