Why are random characters missing in my Debian virtualbox

Its almost certainly a VirtualBox issue. I would suggest changing the graphics controller to one of the other options. I have my Linux VMs set to VMSVGA as that has had the fewest bugs.

If it makes you feel any better, VirtualBox 7 has had quite a few regressions specifically with the graphics stack and has caused problems with all of my VMs. My Windows 11 VM only rendered a black screen until 7.0.10 and all my other VMs are still using old versions of Guest Additions because of the instability with the newer versions.