This kind of design and construction has a long history, and it is difficult to trace the land procedures. According to the relevant laws of land contracting in China, if someone here has gone through the certification procedures, you have the right to use it legally.
Even if the land formalities are lost, they cannot do whatever they want. If you can't provide more evidence to prove that the land use procedures are complete, and if they can't provide conclusive evidence to prove that you have no rights and they have rights, then the land use right still belongs to you. Specifically, you can consult a lawyer and file a lawsuit according to law to safeguard your legitimate rights and interests.