If the OC is also reporting a DOFD, then FCRA 623(a)(5) sets that as the DOFD that must also be reported by the debt collector.
Debt collectors must obtain the DOFD ftrom the OC, and thus reporting of a different OC than that shown as having been reported by an OC is contradiictory.
Thus, you have a prima facie case of improper reporting of the DOFD by the debt collector.
Once a furnisher has verified the accuracy of reported information, they can dismiss a second and subsequent dispute on the substantially the same basis as being "frivolous ir irrrelevant." You have two options for further action.
Since the facts seem to clearly prove your point, you can file a formal complaint tihe CFPB, and hope they force a resolution.
You can also or additionally bring your own civil action based on their lack of a reasonable investigation of your dispute, which they verified nothwithstanding the clear showing in your credit report that the OC is reporting a different DOFD.