I'd say it has something to do with how you are gripping the gun. As far as I've noticed in my years of shooting Glocks and HK's The Glock requires a bit stiffer grip and more square stance whereas the HK needs a bit lighter touch when it comes to gripping the pistol and a slightly more forward/open stance. I'm not talking a complete change in stance, but for Glock, feet almost parallel and HK's, for right handed shooters left foot slightly forward of right.
Often times people forget to manipulate their legs as well as their grip when it comes to proper grip and stance. The recoil from a pistol doesn't just affect your hands and arms. That energy is transferred throughout your body to the ground. Changing how you stand is very important and I've been able to determine for my own uses how I need to stand with every different pistol that I use. Some overlap, some are the same, but different pistols require slight modifications to how you shoot them.
I'm not saying that the traditional stances taught by countless people some famous, some not are in need of change, just that every shooters body is different than the next so there needs to be some slight modifications to the stance based on the shooter to best utilize what they've got.
As to the problem of the brass hitting you in the face, I'd say that the energy required to "fling" it out over your head or to the side is being dampened by a weak grip on the pistol Now, that being said, I am not there to see it or analyse it, or shoot the pistol myself so I could be flat out wrong. Please don't take that as a personal attack, but I've shot with/analysed problems for/taught many people the small little differences that I've noticed after watching them shoot and putting it on film for them so they can see what is happening during recoil. I haven't seen alot of video posted on this forum and that may just be by chance, but I film alot of my shooting so that I can watch myself and see what I'm doing wrong. It helps alot. I have a little camera that'll run 60 frames/second and that is worth a million bucks to me, because I can slow it down a bit and watch what is happening. I'd like to get a camera capable of 200+fps but those are more spendy than a fine firearm. If you have the ability to film it, I'd say it would be invaluable to helping solve the problem. As always be very careful, use a tripod so as to not put someone in the path of hot brass. (Wives don't like being camera people, just for the record hehe)