It isn't a bug, it's an unfortunate side effect of the way in which the modifier that Volley Fire applies to Rear Echelon Troops works.
The Stuart, when using its ability can do extremely well against Pumas, especially if backed up by the Capt or an AT gun - both of which will be around if you have a Stuart on the field.
Agreed.
The Stuart and the Puma fight on even terms. They have different advantages.
The Stuart has the Shell Shock ability which stuns the Puma and increases the rate of fire of the Stuart temporarily. Stuart beats the Puma if it gets this power off any even a slight bit of help.
Puma has range and better DPS. If it gets a good few shots before the battle begins it will win against the Stuart.
In close quarters combat the Stuart has the advantage. In large open fields the Puma has the advantage. They can fight each other competently.
There are huge issues with the US forces.
Volley fire and the M15 AA Halftrack are chief.