This was tested on Firefox (snap), Zen browser (Flatpak) and Pale Moon (native/tar)
Whenever I try to save a file on my Nextcloud, it doesn’t seem to go through. I can access the share just fine, however.
Opened a bug just in case
https://bugs.kde.org/show_bug.cgi?id=500153
Reason why I figured it was portal-related is because, on Pale Moon, I had to use Plasma’s file picker to access the webdav (on the ugly GTK file picker).