I’ve just tried out what you’ve done and can confirm the sweep.
The issue is based on the same error I already posted here: Soundbox 1.2 audio output broken with ping pong loop
If you have a look at the samples you’ll see that they’re also loop via “ping pong” which causes the bug. As soon as one playbar hits the end of the loop and moves backwards, the bug is triggered.
The sweep happens with the Dark voice tension loop sample from the Spectra pack.
I hope this get fixed soon because other patches from that pack are also broken.
EDIT:
I’ve just discovered that if you add a bit of X-Fade, at least 1%, the bug isn’t happen anymore.