Mob breeding in 1x1 cell no longer working

Since the 1.67 update, any farm that I had where I breed passive mobs in a 1x1 holding cell no longer produces offspring. Hearts appear, and I do not receive any mob cap limit message, however, no offspring is produced. I have created a new world in creative, and created a simple 1x1 cell two blocks tall. I get the same results, love mode enabled, no mob cap message, and no offspring. As soon as I break one of the sides while the mob is still in love mode, the offspring is produced. Any other sized holding pen works just as expected including a 2x1.