Mastery works just fine on CL if you actually hit the right number of mobs. The reason it has a reduced chance to proc is you'd get triple the value otherwise. The correct way to look at Mastery chance is per cast. With one target, you have 1/3 chance per cast, with 2, 2/3 chance, 3 you get the full chance as well as the full damage potential. Glyphed, with 4 you have a 33% INCREASED chance, and with 5 a 66%. Additionally, Mastery is upping the chance for your T15 2pc and 4pc to proc.
There is a debate because it isn't as simple as you are making it out to be, and the actual fights don't occur the way in which you are simming them. I did see your original post, and I question the validity of your results as I don't know what Sim variables you ran or what criteria you used, good practice is to link to the results themselves, not just your interpretation of them.