Please do not do this... Do not load a projectile while filling. You should cock it, but don't load a projectile...
That is what helped me having a leak thru barrel when filling issue, granted on my Epic Airguns Two, and it is the suggestion i got from them when i asked.
I tried to cock rifle too, CUZ that is also what í have done / must do with my FX Maverick when it have been completely degassed.
I also tried opening regulator ( CUZ i am shooting a 700 mm barrel 4.5 mm rifle so REG pressure not super high, often the lowest 56 BAR i can set it to in order to be able to shoot lighter 10 grain pellets at not too fast speeds )
I had this issue for a while, first the valve would shut and the rifle then fill just fine, just had to waste some air for a few seconds before it was able to help valve close.
I also tried several valves CUZ i have spare of those for the Two, but that did not help.
Also just had a problem where the valve will start to leak after a few shots, or appear to be fine holding air for days before range session then shoot a little, and get home and in 1 day all pressure was gone, that also resolved itself in some to me unknown way.
Of course when i did do that, i did de cock the hammer before filling, so little chance of chambered slug getting discharged, well at least fast like in a shot, if the leak was permanent the pressure would of course slowly build and press the slug out the barrel, but i assume this would be none lethal.
I was not worried about making holes in my apartment.
PS: this filling issue also resolved itself again, how i have no idea.
ATM i have a love / hate relationship with balanced / assisted valves,,,,,, my Two though shoot very very nice and i would not be without it.
Also fairly confident if i get a new bulpup rifle to replace maverick it will be RTI Mora,,,,,, especially if RTI can do something good with the 4.5 mm barrels