I may be misunderstanding the nature of the question because I haven't played with PP/Aux stuff yet...
But, if the Echo button on the target track (so the Aux track) is still engaged and the source tracks are still routed to it then it would make sense that you would get the input sound from the source tracks ON TOP of what you just recorded on the Aux track once you hit play. If you turn off Input Echo on the Aux track then the source tracks are no longer coming into the Aux track and all you hear is the recorded clip on the Aux track (which will be normal volume/quieter).
Kind of like if you had recorded a guitar track, left the input echo on then played the part again on your guitar WHILE the part you just recorded played back. It would essentially be two of the same thing being heard at once and thus increasing the volume.
However... as I said I may be misunderstanding the nature of the question or the nature of the new Aux/PP tracks themselves.
Cheers.