I had a similar problem last month at the maesot border, I wanted to cross ending a retirement extension and enter visa exempt. They kept me in their waiting room about an hour checking every stamp in my passport, they finally said there was a problem with one of my stamps not being recorded in their computer from two years previous, (how did I get extensions since that time ?) I showed them that the stamp was from their office, and how is it my problem.? Finally, they somehow magically fixed whatever the problem was, Then asked me why I didn't have a Myanmar visa ? I told them I don't need one.