I have both the ballistic nylon Pilot and a black halcyon Pilot that replaced my black dyneema/original halcyon Pilot a few weeks ago. I use the halcyon Pilot as my EDC bag and the ballistic when I travel and although the later is obviously sturdier and slightly heavier due to the material, I have not had problems with either version having the rear flap sagging.
I have used the original halcyon bag for a while now and have not had issues with the rear pocket unless I take my iPad Pro 12 or MBP 13 cache out of the bag; even if the cache is empty it seems to stabilize the structure of the bag and stop other items from digging into the back. I guess what I am getting at is that the rear pocket only loosens up if I pack the bag towards the bottom with nothing to give it rigidity like the cache (not as much an issue on the ballistic but it will still happen if you overpack it).
Hope that helps