Ahhhh
I think I found out the issue that you're describing. If you do not assign HTTP (or whatever) to a higher or lower priority queue, it is left to the default, which doesn't automagically assign ACKs, due to no rule being there.
If you want that behavior though, the easiest way I've found is to create a rule on the floating tab, with something along these lines:
Action:queue
protocol: any
source: any
destination: any
queue: qACK/qDefault
Then, move this rule to the very top of the floating tab before all other rules. All traffic will then have access to the ACK queue by default, and it will allow other assignments to change the traffic to another queue is needed.
The bad news is that there will be an extra rule to process for each state. Under light use, this won't be a problem, but when you get to heavy business rates, it could choke the CPU.