Guest additions from guest additions CD can cause issues as per: VirtualBox Guest Additions - #3 by anontor //cc @anontor
How did you install VirtualBox version 6.1.8 guest additions? Debian package or ISO / CD?
Creating a Debian backport package of VirtualBox version 6.1.8 guest additions for Debian buster hard as per:
Missing dependencies on Buster for Virtualbox 6.1.6 - #4 by Patrick