I have tried this in both MBT and WW2 (using your opponent pair of USA v Japan in 2/42 for the latter).
In over 100 test purchases, this problem has
not appeared at all. This was using the battle generator of course, I tried assaults, advances and meeting engagements for variety.
It's definitely borked, as the USA has on-map mortars etc in General Support (only off-map arty can be so).
However, the campaign save is useless for debugging since it's at a point after the AI purchase - do you have a save prior to the game purchase?. (but there is no difference in the AI buy routines between a campaign and a generated battle - me scratching head here!
)
Andy