Breach seal may be bad, my venom did that a while back. It did make a loud crack like a 22 when I shot it. Put a new in and everything was back to normal.
If you decide the backwards pellet was the problem, a coaching session on being very careful about putting the pellet in correctly is in order. When I was teaching boy scouts gun handling and shooting. I used pushups to give them time to think about their mistakes. Down & give me 20 was common! They thought it great fun to watch their buddy do penalty pushups!! My son preferred pushups to my standard for him of a rap tin the head with a knuckle. Didn't have to do either very often. An experiment to try, if you are wlilling to. Put a pellet in backwards intentionally with gauze around the action and see what happens. Thinking about it, the chamber may have a step to hold the pellet in the chamber properly and to upset the pellet into the rifling. A backwards pellet might hang up enough to give the effect. I am visualizing something like a rifle or pistol chamber. I haven't really tried to look to see if they have a step like that!