Quick question - why can't Guderian trace supply to Rundstedt (who is in supply)?
I imagined it would be From Guderian (Supply Range 2), tracing one to the rail NW of him. Then it follows the rail to Rundstedt, who is in supply through Kabul and another HQ to Russia.
Is it that Germany can't trace through Japanese territory? Or is this a bug?
No, Guderian cannot use a Rail Supply path to Rundstedt, a secondary supply source. Once you start tracing supply by rail you have to trace it to a Primary Supply source somewhere. Of course, any supply path (including a rail supply path) can be traced overseas, but only once.
That said, I beleive Guderian should be in supply!
1 hex to the Rail, NW of Guderian. Then by rail via Delhi to Calcutta. Then by Sea via Bay of Bengal, South China Sea, China Sea, Sea of Japan, to Vladivostok (2nd Hex). Continuing by rail from Vladivostok, all the way back to Germany.
The reason, I suspect why it isn't working, is perhaps it's trying to use the same rail path as Rundstedt. (hit F8 and you can check this). It, for some reason, lands it's sea supply leg in Foochow, and then uses its 4 hex limit overland to a rail hex near Nanchang! If the program is trying to use this same supply path for Guderian it will be to far, as Guderian only has a 2 hex supply path limit due to rain. I suspect the code has an internal limit as to how long (or how far) it checks for valid supply paths, for reasons of game speed and performance, and given your particular situation its exceeding this limit.
I agree that the problem is an internal limit, but I suspect the problem is the Transsiberian railroad. MWiF does not like tracing supply the whole length of it. I have had problems with the US not being able to trace to a French city along the railroad after Germany was conquered.
I thought I knew how to play this game....