We have a quit claim deed for land that was abandoned by the county, can a government agency revoke that deed?

The land which was quit claimed is adjacent to ours and has been used exclusively by our family for generations - our front porch and lawn actually rest partially on that land. The bi-state agency (TRPA) says that the quit claim is invalid because they were issued after they received their charter and now insist more than 21 years later that the land was not property subdivided before it was quit claimed. Our property value has been assessed with the inclusion of this strip of land - do we own the land or does TRPA have a leg to stand on?