Official comment
@Dimitar > Unfortunately that's not the way slip mode works; slip mode puts the primary audio playback as the track itself, any other functions allow the main playhead to continue in the background while you perform secondary actions, such as looping, scratching, reverse and hot cues. I've created a graphic to give you a bit of a visual:
<snip>
To answer your specific comments:
1) Those actually change the loop into a kind of roll; this is a "slip loop." It re-samples the audio at the position you tap (change) the length and starts the audio looping from that position while you hold the button. If you manually create a loop or use the 4-beat loop button (not the screen), it will loop - you can press the 1/2 or 2x buttons to change the length while the slip continues in the background.
2) If you are reversing while in a loop during slip, the loop itself will reverse, and the slip will continue in the background.
3) If you created an 8 beat loop at the 0:00.00 position of the track and left that loop running for 1 minute, then exited, the playback position would be at 1:00.00, not at 4 seconds (or whatever). This is because the play position continued along, slipping beneath the loop. This is perfectly normal.
4) You're in a loop - of course it stays where the playhead is; the loop is what's slipping, not the vinyl action slipping the loop. Because the loop is a slip function, not the scratching within the loop, it won't work the way you're asking. Sorry!
From my comprehension of what you're asking and what the players are capable of, this isn't a bug and while I understand the possible usefulness of an "in-loop slip," I don't see that as something the engineers would add in a firmware update to current models as it fundamentally changes how slip mode operates.