Carbon builds where it goes. Where it travels to and is stopped.
The higher your end clearance the lower the angle for gas to egress from case mouth backward to between neck and chamber neck. All the way back to shoulders if your neck clearance is high enough and you FL size necks and/or run low pressure loads.
When the case mouth expands to chamber it shuts off flow momentum right there, and that is where carbon builds. Right at the case mouth, regardless of where you put it.
Now, with tight end clearances, you've established very high angles for gas to work it's way back through. Add rational neck clearance and partial neck sizing, and the case mouth will seal before much if any get's past it, and little if any backward momentum has developed. A buildup here drops to very manageable/harmless residue.
If all your cases are trimmed way short of chamber end, you will have problem carbon buildup(if unmanaged). So with further reload cycles you'll need to mind your trim lengths with more diligence. Otherwise a longer case could chamber with interference fit and result in a pressure spike. Easy enough to manage also, but easier to stop causing it to begin. Just stop trimming so much.
A wildcat of mine has fitted necks, partially sized, and trimmed to within 5thou end clearance. I run at SAAMI max. There is zero sooting on necks, and zero carbon left in the chamber(not even at the chamber end). Here, there is no backflow, only what I imagine as brief expansion that is sucked right back to the bore with bullet release.
