Bose QC35 noise cancelling was gimped

Both generations of Bose's QuietComfort 35 headphones have long been some of the most popular noise-canceling cans around, and rightly so. They're known for delivering both excellent sound quality and first-rate noise-canceling that can keep the world out when you want some peace and quiet to enjoy your tunes (or audiobook/podcast/movie/game).
However, after Bose recently rolled out a round of firmware updates to the original QC35s and more-recent QC35 IIs in June, some users have complained that the noise cancellation feature seems to be less effective than before.