Yes this is expected behaviour, if you are viewing the user's membership with ADUC. The membership of a user is stored in the memberof attribute, as DN based on the domain A name context. When domain B try to resolve the members it will pass the DN to the domain B to be resolved. However, while you have a trust, this doesn't provide an LDAP referral, so the domain B domain controllers will not be able to resolve the DN and will return the following error:
Error: (0x0A) A referral was returned from the server, Server Error: 0000202B: RefErr: DSID-03100838, data 0, 1 access points
ref 1: 'w2k12.local', Ext Error: (8235) A referral was returned from the server.
Referral: ldap://w2k12.local/DC=w2k12,dc=local
So the membership can't be displayed. The reason why the domain users is displayed, is because this is the primary group, which stored in primaryGroupID attribute, and this is stored as a RID (part of the SID) which can be resolved by the SIDtoName API, as it will use the trust to resolve the name.
Gary.