Certainly not something I’m an expert with when it comes to GBBRs, but as far as I’m aware mechanically there is very little difference between GBB, HPA and AEGs when it comes to loading/loading nozzles.
A BB running down a barrel with the hop off is entirely normal in many cases. It’s not so much to do with the ‘air seal’ as it is the lip tension in your hop unit. Some hop rubbers will have more tension, some will have less. I could be entirely misremembering but I believe Maple Leaf buckings have more lip tension than most stock buckings... but I’ve been through so many different brands of bucking on so many different RIFs at this point I get lost. If the rubber has a lower amount of tension on the lips it’s often the contact patch alone being depressed into the chamber that prevents them rolling down of their own free will. Conversely, too much lip tension can wear out/break a loading nozzle prematurely.
You have all the tools available to you to verify your results. Rather than speculate ‘I don’t think this happens with the stock components in place’, check them. Don’t be lazy, reinstall the original parts and see how it’s performing.