In the documentation (comments) of the configure directive
/etc/adduser.conf), it says that:
# If SETGID_HOME is "yes" home directories for users with their own group the setgid bit will be set. This was the default for versions << 3.13 of adduser. Because it has some bad side effects we no longer do this per default. If you want it nevertheless you can still set it here. SETGID_HOME=no
What are those side effects?