I know you know your stuff, but are you sure the other units weren't bumped out of the used units?
It is still possible to lose low casualty units when you have a mix in units used, but it should be a lot rarer (and has been in test over here, every time we've looked into it). Imagine every time you lose a unit, the game rolls a 10-sided die. If it comes up 1-8 you lose a high casualty and 9-10 you lose a low casualty. Bit more complicated than that, but that is the gist.
That bug was thankfully fixed a long time ago.