Not really a problem, I take all the Duchies I can and leave just one or two 1 province Duchies in any area, because I need someone to discover and spread technology in most areas. I have my heir and I play a game of tag, I take a province, hand it over to him, have him build me a royal post and give it back to me. That way I have all these counties that give me only 20% income + mines + breweries.
The reason for the switcharoo is that you can't build royal posts or anything for that matter, once your income hits zero, which is what happens when you have a large demense, greater than your limit. Not sure if it is a bug, I will post it, see what the staff says.