In the documentation (comments) of the configure directive
/etc/adduser.conf), it is said that:
# If SETGID_HOME is "yes" for users' home directories with their own group, the setgid bit will be set. This was the default for << 3.13 adduser versions. Because it has negative side effects, we no longer do it by default. If you wish, you can nevertheless define it here.
SETGID_HOME = no
What are these side effects?