You’re not the only one. It was mentioned earlier in the thread and I did have the same issue (amongst other bugs) last time I tried the repo version of Krita on openSUSE Tumbleweed. I do miss the native KDE file picker in the app image though, and the ability to use system themes was fun. ^^
I’ll add a notice to the first post about it.
EDIT: I tried the flatpak build of Krita and it looks to be working fine as well, and the native file picker becomes a Gnome one. Also tried the snap but it is outdated, 5.2.2, and crashed on me when creating or opening a document. It comes with the KDE file picker though. 