It's not a bug. The processor has to process one mixer "first", and then the next, and so on. If you want a priority mix, it must be higher in the list than the secondary mix. Mixes are processed serially, so you can't have a later mix operate an earlier one.

You can't get away from the order in which things get processed.

You don't have to reset memory to set up mixes. We mentioned that we did it so that we'd have a 'clean slate' to work with, which made programming easier for us in the example.