Just to clarify, ALL oil & gas mineral leases stay with the lands regardless of whether there is a change in ownership. They will expire or relesse only under the terms of the lease.
It is no different than if you own a building and have signed leases with tenants. If you sell the building, the leases don’t go away. They stay with the building and expire under the term of the lease.
That “shady, carpet bagger” land agent was very likely just following the law. Any attorney would have advised them the same thing.