Cannot download to VirtualBox shared folder

[thargoid-elite]

System details:

• Xubuntu 20.10 both host and virtual machine. Retested with Kubuntu 21.04 virtual machine—same issue.
• VirtualBox 6.1.22.
• Tested granting both full system access flatpak override org.qbittorrent.qBittorrent --filesystem=host, and access to vboxsf mount point.
• VM user is a member of vboxsf, and can read and write files to a shared folder.

qBittorrent can open torrent files from vboxsf shared folders, but an attempt to download a torrent to the shared folder results in Permission Denied.

qBittorrent installed from PPA does not have this issue, that's why I'm reporting it here.

Strange details:

1. Download some blocks from a torrent using qBittorrent from PPA, then switch to Flathub version, and recheck. After recheck, Flathub version can continue downloading the torrent for as long as it does not result in the creation of new files. Downloading new blocks to already existing files works, but an attempt to create a new file results in Permission Denied.
2. Remount the shared folder using VirtualBox GUI. After remount, attempt to recheck a torrent from a shared folder using Flathub version. Torrent directory will get nobody:nogroup owner and group, which cannot be reset with chown root:vboxsf. Access to directory can only be restored with a shared folder remount.
3. After remount, recheck the torrent using a PPA version. After that recheck, Flathub version can continue downloading like in the point 1.