Welcome to metallic reloading. You are now learning why many reloaded will tell you primers are a very bad indicator of pressure. You are misreading what you are seeing, those primers are not flattened. Like said, the corners are still round. The primer will fill the pocket when truly flattened. While I'm a huge fan of HS-6 I use it where it will work best. HS-6 preformes ỳ much better at the top end of the pressure curve, not how light you are loading it. I like W231/HP-38 for all but the hottest 9mm ammo I load. Not only are you loading very light you are lessening the pressure further by lengthening the COAL. You are loading way too light for HS-6 IMO. HS-6 is a hard to ignite slow ball powder and it benefits from use of a magnum primer. Years ago the people who wrote the load manuals did specify magnum primers with H110/W296, HS-7, HS-6 and the like, even in cartridges like the .38 Special. Today the just tell us to use a standard primer with standard cartridges and magnum primers in magnum cartridges.