Along the way to implementing the ad hoc how water recirculation, I reflashed the Atom Matrix multiple times. Most of those times, the device lost its mind and turned on all the LEDs at full brightness. Sometimes it would recover if I cycled the power, and sometimes it did not. 

I spent a lot of time fooling around with different past versions of ESPHome (which I run as a docker image, so trying older versions was pretty simple). I also did a little Internet-style research on the FastLED library, which is what is used to control the animations.

Eventually, through trial and error, I hit upon the answer. I display a rainbow animation at boot-up, just as a signal that no flame status message has been received yet. I added a few seconds delay before displaying that initial animation. My hypothesis is that I was previously interfering with the initialization of the WS2812C controller for the LEDs. Why this suddenly matters now is probably just other subtle changes in how fast things happen during the ESPHome start-up sequence. In other words, it was always a problem, and my luck suddenly changed