64bit busybox seems to ignore umask, all dirs created by mkdir in busybox's ash are 1777 % busybox ash -c 'umask; mkdir asd; ls -ld asd' 0077 drwsrwsrwt 2 piotr users 4096 Feb 28 23:43 asd tested with gentoo's busybox (bulit with glibc) and busybox bulit inside Aboriginal Linux (uClibc) the i686 binary looks to work fine.
Already fixed by commit af36ba206f7cf0eef77a82af741766a2d03c51ad