It's not your imagination! Blue has finally reassured transmutation-specced alchemists that alchemy transmutes have indeed been suspiciously light since patch 2.3. In a post in Blizzard's Customer Service forum, Pavonum comments, "Ah, good question! Based on the notes I've consulted, it seems that Primal Might is currently the only transmutation proccing extras at the intended, higher rate; all other transmutations are proccing at an unintended, significantly lower rate. I hope that provides some clarification. :)"
A new sticky in the Professions forum updates players on the status of this issue. Naethera notes: "As of the 2.3 content patch, there's currently an issue with Transmutation Mastery by which the percentage chance to receive additional results from transmutations - aside from Transmute: Primal Might - is lower than the intended value. This is likely contributing to the dearth of procs you've observed, and I'd like to offer my apologies for any consternation that has arisen as a result of the discrepancy. I'm happy to report that this matter should be resolved in a future patch, but your continued patience in the interim is appreciated. :) "
That's a lot of blue :) to offset a lot of alchemist :( -- but the problem should be resolved soon!