Well the bug never occurs when one is defending (at least I have never had it happen). From my test it seems that the error is linked to the fact that:
the "initial" stockpile that the units use is drained before 0:00 thus the units next day are -20%.
I havent noticed any time dependancies - ie. it doesnt matter if you are moveing 1 week or 2 month - it still happens.
F
EDIT: Well I did some more tests and here is a wierd result. Start the 1939 scen. as japan . Cancel the convoy from Hiroshima to Koshu (the stockpile place) and change the convoy from koshu to hiroshima so that it ships oil and supplies. After a while you should have 0 supplies left in koshu. Meanwhile order an attack from Wuhan to xiangfen. when the troops arrive they are still in supply. Furthermore although you have 0 supplies in Koshu and no convoy running to mainland China you can still resupply units... And suddenly all your troops are in supply. Create a new convoy with supplys to koshu and the bug is there again.
Creating a new convoy to shanghai with supplies does create the bug as the pool moves to the main supply depot (koshu). however it seems that if you set up a convoy to shanghai that transports supplies, but is smaller than the convoy that is russing from koshu the supply depot does not merge + you do not have the -20 penalty. However the convoy from koshu to Japan must also transport supplies. so in effect you are dumping the supplies in one territory and picking them up in another. None the less china is still in supply.
this leads to three thoughts:
1) either the map is mismapped.
2) the convoys are screwed.
3) or the convoys are calculated wrong
or maybe all three.
Anyways if you are playing japan the work around this bug is to:
1) Cancel the convoy to Koshu.
2) add to the convoy from Koshu supplies and oil so that the supply depot is drained to 0.
3) cancel the convoy from koshu to mainland japan so that it does not take supplies and oil
4) dont send convoys to and from one depot in the future..
should work - if not let me know
. mainland china seems to be somehow magically resupplied..
enjoy!
F