That is very possible since that DHCP server..on your old domain, maybe be giving your clients the address to that dns server on the old domain instead of the new.

Have you tried to scavenge stale resource records? Maybe it replicated data from the dns servers on the other domain. Also check your fowarders, by default they are set to forward to all other domains. But you might want to turn that off for the new domain. Also, to see a move advanced log of group policy apply issues on your client machines. If you go to windows\debug\usermode\userenv.log. Look in there and see what common errors your getting. It will tell you if the client machines are even seeing the GP. Hope any of this helps.