A new folder is created with default setgid bit set to S

I have two Ubuntu servers, both with umask 0002. On one of them, whenever I created a new sub-folder in a folder named public, the new sub-folder has the setgid bit set to S. On the other machine, it is s.

ls -al ./public shows the same filemodes on both machines:

drwxrwsr-x+ pi grp1 4096 Nov 27 15:36 .
drwxrwsr-x+ pi grp1 4096 Nov 14 21:56 ..

Which setting creates this difference? How do I make the S one have new folders created with s?