This is the same issue mentioned in bug 9052. I have tried the same test case with samba3.6.12 patched server. And here is the behavior..
setup and configuration:
created a User: Hemanth
created a Domain local group: east_users (made user Hemanth as part of this)
Made samba server joined to WEST domain.
# wbinfo -a=EAST_USA_MD\\hemanth%Password@123
plaintext password authentication succeeded
challenge/response password authentication succeeded
# id EAST_USA_MD\\hemanth
uid=1792541994(EAST_USA_MD\hemanth) gid=1792541185(EAST_USA_MD\domain users) groups=1792541185(EAST_USA_MD\domain users)
Here it did not list the domain local group east_users.
But bug 9052 says this is fixed as part of 3.6.6.
Entered Some wrong data..