I actually had a similar thing happen with gvfs-fuse (Google drive). It was a bug in gvfs using the quota usage of a file as it's file size (because libgdata didn't provide a method to get file size), but I was using a file shared to me so it had zero quota usage.

All of which is to say I would bet on something in the webdav-nextcloud line being at fault instead of keepassxc.