This might be fixed in VirtualBox 6.1.36. And even more likely so in Virtualbox 6.1.38 which is already available from upstream VirtualBox.
The problem is…
…Debian fasttrack at time of writing is shipping only version 6.1.34 and I have no idea when or if Debian will update VirtualBox.
Installation of VirtualBox from VirtualBox.org Repository (6.1.38) might be possible but it’s cumbersome and quote:
Note: after upgrading the VirtualBox host version, this can cause issues with the guest VMs such as broken VM size adjustment (full screen) [2] unless the VirtualBox guest additions are also upgraded inside your virtual machine (which is not covered in these instructions).
related:
Previous VirtualBox Integration