I've noticed it really depends on many things. For me, Tena Slip works fine and does not break down or bunch badly, even though I am in the summer heat of Eastern Europe at the moment. I've heard that the Tena brand in US is of inferior quality comparet to the European make, though. In my use, for some reason, pull-ups clump worse than slips. It may be a matter of individual differences in body size, levels of activity, type of work (sedentary vs. physical - I work in an office currently) etc. I've also noticed that the softer the padding, the worse it breaks down: the mass is probably fluffier than in more tightly packed and dense diapers and when exposed to heat, moisture and movement the pad breaks into uncomfortable and nasty balls. Another contributing factor is the underwear covering the diaper. Even though some prefer to wear just the diaper underneath the clothes, a snugly fitting pair of underwear helsp to keep the diaper better intact, probably as it has less room to move about and have friction and stress directed to it from pressure and body movement.
However, the brief or pull-up choice is very brand dependent: AMD slip was light and relatively cool, but broke down very quickly. I walk a lot, both about the town and in rough terrain, so this might contribute to the bundling and breaking. So, if opting for a lighter/cooler protection on better bladder days I would choose a molicare or tena pull-up over AMD slip.
I'm happy to hear you found a workaround that helped to keep the bunching at bay.

-Ü.