I regret that I am not going to be able to diagnose your problem without being there, especially as you seem to know what you are doing. My guess is that it is one of the following:
1) Something at your end, probably cabling or ground related, or maybe a peculiarity of your amp. This is most likely, as the nature of PRE/POST is that if almost anything goes wrong, you get mains hum or feedback.
2) A hardware problem in the unit. I've not heard of anything that will do as you describe, but there is always a first. If you think that this is the case you should send it in for service ( email@example.com or your dealer ).
3) It could be a software problem, but none of the many many people using this feature have reported it.