For some reason, the program thought Umezu in the following set up was out of supply. When I saved the game and reloaded it, he was back in supply. (So I don't think a saved game here would help.) Immediately prior to the problem, Umezu was one hex to the south-east and in supply. He moved, and went out of supply. (Umezu's supply chain goes back to the Yangtze River Road. There are convoys back to Japan, and they weren't being touched.)
The bad thing here is when I move units and don't notice that they are disrupting as I move them. (If I were observant, I would see the DSR indicator on the move icon, but I am not observant.)
Changchun once went out of supply in a similar fashion; a load and restore fixed that problem, too.

I thought I knew how to play this game....