While looking at the code around tab-complete.c, I found the ordering in words_after_create array is not correct for DEFAULT PRIVILEGES, which been added under below commit:
commit d7d77f3825122bde55be9e06f6c4851028b99795 Author: Peter Eisentraut <peter_e(at)gmx(dot)net> Date: Thu Mar 16 18:54:28 2017 -0400