This was fixed as described in:

http://bugs.gnu.org/14371

I don't take that text in the standard literally.  In the hypothetical "POSIX plus abandoned POSIX ACLs" standard that we're implementing, I feel certain it would have been changed to account for default ACLs.

I really doubt that a user who applies a default ACL to a directory would then rely on mkdir honoring the umask in preference to the default ACL.  But if you're genuinely worried about breakage, I would be happy to have a new option, or preferably a new environment variable that I could set globally.  This issue causes me pain every time I install software from source: since I use umask 0077, installs into a shared directory with an appropriate default ACL are unusable by others until I go and change the permissions manually.

I would like mkdir -p to observe the default ACL in preference to the umask (as nearly all other programs do) when creating parent directories.

To reproduce, run this in an empty directory on a filesystem with ACLs enabled:
umask 0077
setfacl -dm u::rwx,g::rwx,o::rwx .
mkdir foo
mkdir -p bar/baz
ls -l

Actual result:
total 0
drwx------+ 3 matt matt 72 Apr  7 11:32 bar
drwxrwxrwx+ 2 matt matt 48 Apr  7 11:32 foo

Desired result:
total 0
drwxrwxrwx+ 3 matt matt 72 Apr  7 11:32 bar
drwxrwxrwx+ 2 matt matt 48 Apr  7 11:32 foo