Welp, happened again in Fitness+ this morning! I tried using the remote app on my phone, the volume buttons do indeed change the volume bar on screen but they do not change the actual volume on the Airpods—full volume and lowest volume sound the same.
What did work, again, was playing part of a Foundation episode. Weird.
What I think is happening is some audio content is causing a bug with my AirPods Pro (obviously) but playing a proper Atmos audio (not sure if Atmos/Spacial Audio music tracks will do it) resets the state of the AirPods and the start interpreting audio correctly again.
If it happens again, I’ll try “fixing” it with different audio sources and report back. Hope this helps someone else and whatever is causing this gets fixed in the next update!