That is very interesting as keyholing with the Lee 120TC was my initial problem in a G17 too. It was detected at a GSSF match when inspecting targets. It appears that the rounds were just tumbling in the general direction of the target. I sent that mold to eBay heaven and replaced it with the Lee 356-125-2R and I have no sign of tumbling. My guess was that the G17 just wasn't getting a good grip and wasn't getting it spun up fast enough. I also had horrible leading with it too.
I'm glad you've found something that works. The 9mm can be quite the challenge to get it to run well.