You're correct that powder position can play more of a roll than people often credit it for doing. The examples I have personal experience with both come from rifle shooting, but they still indicate the combination of ball powder and poor case position is a recipe for trouble.
Without laboratory measured proof, here is what I suspect happens in the two most extreme cases for your gun and load: shooting straight up and shooting straight down. When the gun is fired upward, the powder is at the back of the case and the primer flame blows through a collection of grains. This not only ignites the grains close to the flashhole, but puffs them out in a cloud that spreads the flame and, exposing their surfaces, makes the grains in the cloud ignite more easily and completely over their surfaces. The grains that didn't puff up into the cloud are still lying in the form of a crater rim around the flashhole. These are ignited on top and burn downward. What moves forward from the case mouth are the bullet and the cloud of hot gas.
When you shoot straight down, the powder lies in the form of a plug behind the bullet base. The flame from the primer never actually blows through any of the powder mass, so only the far end of the flame touches the powder and starts the "plug" burning from the rear forward. So the primer actually ignites a smaller portion of the powder initially. By the time the back of the bullet clears the barrel-to-cylinder gap, a good deal of un-ignited or partially ignited powder follows behind it. The opening created by the barrel-to-cylinder gap offers a pressure vent at a moment when a much higher percentage of remaining unburned powder is still trying to build pressure than was the case in the upward shot example. Because of its position, a bit of the powder may well be blown out through the barrel-to-cylinder gap, effectively reducing the total charge.
You could test that last notion by placing some white paper on the shooting bench under the gun and firing it held sideways like gang members do in the movies. The spray from the gap will show up on the paper. See if it is worse when you've tipped the gun forward before firing?
Here are the two rifle situations I've encountered: M72 30-06 match ammunition gives 80 fps difference in mean muzzle velocity depending on whether you tip the gun down or up just before firing. The standard 47.2 grain charge of 4895 only fills the case about 85%. I'd seen this reported in magazines and tried it myself with a Garand, measuring velocity on an Oehler P35. Sure enough I got just about exactly 80 fps variation in MV.
The situation gets worse with ball powder. Ball powders are championed for their lower burning temperatures and better metering consistency from powder measures. But the shape of the ball grains and the small spaces between them impede hot gas flow much more than stick powder grains do. The result is a much higher dependency on consistent ignition pattern to keep their pressure v. time curves consistent.
I proved this to myself with an M1A years ago. I'd seen some photos of primers fired against a black background in case heads cut from cartridges. These showed de-burred flashholes resulted in a much broader, more uniform flame ball. Cases with burrs sometimes shot flame to one side or the other of the case, and the flame shape varied. I found de-burring case flashholes would cut 100 yd group size from 1.5" to 0.75" when loading AA2520 ball powder, a recommended ball powder for .308 match loads. With any of several stick powders I tried I could see no improvement from de-burring flashholes. Apparently the bigger spaces between stick grains allow them to ignite pretty well regardless of the starting flame
Another approach is to use single-layer newspaper wads pushed down over powder charges to hold them back against the flashhole. For a straight case, you just sharpen an old case with your chamfering tool and use it as a paper cutter to make the wads. This is the kind of thing that delights bench rest shooters but makes pistol shooters groan. Just a matter of the volume difference in the number of rounds they fire. Too much work for handgunners.
You can also try de-burring flashholes to see what difference that makes? Meanwhile, it would be interesting to see if your load would get stuck in a barrel with no barrel-to-cylinder gap, like a Thompson Center barrel?