What happens to mineral rights if property is sold when oil and gas royalty payments have been assigned to a deed of trust?

Years ago my father owned 80 acres of land with leased and producing oil and gas wells. He got a recorded exemption and sold some acres with a warranty deed recorded in the county. There is no reference made to mineral rights so as I understand it they would have been included. However at the time of the sale the royalty payments had been assigned under a deed of trust. Would that have resulted in severing the mineral rights during the term of the deed of trust as they paid for and secured it?
I suspect this was understood between the buyer and my father because as far as I can tell the buyer of the lot never claimed mineral rights or received royalty payments even after the DOT was paid off. Perhaps this was included in the sales contract because my father didn’t seem to think that he’d sold the mineral rights along with the land.Then a few years later the lot was sold and the new purchaser claimed an interest in royalties. I’m now being asked to sign a document agreeing to the division of ownership interest including the purchaser of the lot and I’m not sure what to do.

Please consult with an Oil/Gas attorney. I'm not sure you should rely on the opinions from a discussion group that doesn't have access to the information and documentation you should share with someone who could tell you if you have a legitimate claim to the royalty income.

Thanks for responding and I think you’re right. Before signing the document I really need to have an oil and gas attorney look into it.