Hello and welcome to the 10mm communtiy
I've run into this also with P.P. and other powders as well. I just attributed it to two possibilities after playing around with different primers and seating depth.
One, is the powder doesn't like to burn as efficiently at that higher pressure as it would with a lower charge. PP has not impressed me as a powder to use for 100% loads... More like 85-90%
The other thing is that with the increased powder charge you just ran out of barrel to burn all the powder. There is probably a good reason why Alliant lists 9.3grs with a 180gr bullet.
The load my G20-SF likes very much is 9.0 of PP with a Hornady 180 FMJ. Makes for a good solid all-around load. After 9.0grs I run into the same phenomenon. My barrel is only 4.6".