I don't really see an issue with your shooting posture,
@Travis186. The crescent butt plate is tucked into your arm between the bicep and the shoulder. Your rifle is across your chest. You are supporting the fore end just behind the center of gravity. One of the things that I have learned from my shooting is that if the impact (for a right-handed shooter) is low and to the right, I am lowering the barrel as I fire. Such lowering may not even be noticed, but it is happening if the rifle is on target when firing from a bench. The left-hand rest on the torso needs to be more solid. This is a very important step to practice. The drop is likely to be induced by unconsciously pushing the rifle forward to "hasten" the firing of the lock. This is common when using a single trigger that the barrel is pulled down when firing. You may not notice this pull down when live firing. Try the video analysis when firing the rifle. When sighting using the figure 8 technique, try having the trigger release as the sights are rising through the center target.
That's what I'm suspecting as well after it's been mentioned. I'm going to have to get it on video live firing to really see it.
Now then, let's talk about the frizzen resting on the flint after trigger release. You have observed that it happens when you are using a thick leather wrap on the flint but not when you do not use a leather wrap. You really do need a leather wrap. Try using a thin glove leather wrap. In both cases, the frizzen is bouncing back and striking the flint. The thick leather wrap cushions the frizzen and it can't bounce back while no leather gives it more bounce. Either the flint is striking the frizzen so hard that the feather spring can't hold the frizzen open or the frizzen spring is so stiff that it opens enough for the flint to clear the frizzen and falls back. That should be addressed. What's your flint life?
See below, since you're not the only one that has pointed it out. Trust me, the lock is fine. I get on average about 30 shots before a flint needs knapping on this little L&R late english.
What do your patches look like after firing? Holes or tears in the patches should be addressed. Muzzle loading rifles benefit from a smooth even crown. Sharp crowns on a muzzle loading rifle will cut patches. When patch integrity is compromised accuracy on target is lost too. The same is true for holes in the patch from sharp edges on the lands. Some bore polishing may be in order.
Patches look good. I went and hunted a few down just to confirm.