What I would suspect is they are trying to get the property from you and sandbag you with the loan. They probably never signed the loan assumption, but knew you were in prison and not checking the house.

In some states like Texas after 3 years of living there they can claim adverse possession if they have been maintaining the property. I think you need to bring suit against them by the deadline to stop this. What you really need that you don’t want is a real estate attorney to look over what happened and your best course to keep the property / get them out.

Your next best option may be offering cash for keys to get whoever is there to move out with a signed agreement and then promptly making it your residence again. But they may not play nice if they have other schemes.