I have no idea what the reason may be, but perhaps this works as a workaround for you.
You can try to trick out the standard file open dialog using the excellent plugin BuliCommander from @Grum999, which is an orthodox file manager for Krita like Double Commander or Midnight Commander (this is the version I recommend to you because it is already patched by me after the hint of @Grum999 on what to fix, since his last version v0.9.2b has a little bug. But in case you want to apply the patch yourself, you’ll find the fix described here).
If you want to patch it yourself, you will find the plugin and sources in its most recent version v0.9.2b here at GitHub, or alternatively now here at Codeberg. It is probably the last version because @Grum999 decided to archive these repositories.
Here, a screenshot of my setup, more screenshots and a manual-like description of the plugin is found here in the forum, or in the repositories linked above:
Michelist