In scriptmodules/

# Debian unstable is not officially supported though if [[ "$__os_release" == "unstable" ]]; then __os_debian_ver=11 fi

So, I added the following after this:

if [[ "$__os_release" == "testing" ]]; then __os_debian_ver=11 fi

This did not change anything (or using 10 there). So, this is likely a bit more than not detecting the debian version correctly.

The only issue with mupen64plus is that the system will try to compile that by default during installation and this ends up in error.