I very much doubt this one is a bug. It is a very deliberate attribute of the mangonel unit, that could easily be removed with, as far as i understand, no other consequences but making units behind it targetable.
It's hard to tell. We have to remember that there was a time when hunt killed by military units didn't expire immediately and it was initially billed as a feature. Same with not being able to attack behind building foundations and now in order to prioritize units behind building structures and obtain the same gameplay it held for 20 years, you have to click an additional button!