I have been fighting with webdav in dolphin as well. In Fedora Kinoite there is a preconfigured share for nextcloud available and connecting to nextcloud is no issue. But when you want to connect manually via the wizard it’s a no-go.
Recently i moved to kSuite(infomaniak) with webdav support. I can connect joplin and other tooling without issues to webdav but via dolphin it is again no-go.
Therefor i don’t believe it’s working as intended and should be looked into