I tried two different rail modules and they were both like this. I doubt they're changing the frame molds this late in the game so it seems like it must be the rails.Definitely something out of spec there. Gotta wonder if it's a bad frame or bad batch of rails. Hopefully 80% will make things right. I have another one to do... I guess we'll see how it goes for me.
I saw the same thing on reddit. I was hoping to beat the odds.I ran a quick web search for "GST-9 problems" and found some discussion on Reddit about the very same problem you have. I didn't read through it all, but it appears that 80% was claiming that it wasn't a "common problem." Also looked like they did send out replacements though.