I currently have a Summer Hunter Jacket, Summer Hunter Pants, Mountain Backpack, High Capacity West, a B95 as rifle on the back, a Pistol (which is in the vest) and a Firefighter Axe as melee weapon on the back and a stone knife also in the vest. With this set I have 9 (or 10) quick slots available. Besides ammunition I have the B95 on the first quick slot, the pistol on the 4th quick slot, the firefighter axe on the 7th and the stone knife on the 8th quick slot. Whenever I log off from the game and log in again the rifle, pistol and ammunition quick slot assignments are still there but slots 7 and 8 are empty (loosing the quick slot assignments for the firefighter axe and stone knife).
This happens always and is not related to the actual slot numbers. Assigning the items to different slots shows the same result, axe and stone knife assignments are gone, the others remain. I had this also with other melee weapons like pickaxe etc. but I did not see that with firearms and ammunition