They seem to run toward the other cav, and stop while in range, and just sit there while they take 11ty billion years getting into the ready position. In the meantime, their targets charge them and kills quite a few. Just my cav axuilia is about to shoot, the suddently stop because they realize they are too close, and the run away a little bit. Then the process repeats itself, resulting in total loss without inflicting any damage.

Velitites and archers don't seem to have the problem against other infantry of the same speed or slower speed. Cav auxilia should be able to out run the heavy cavs and yet they don't work the same as missile infantry. Cav archers doesn't seem to have this problem...