Thanks to those who let us know of the defect with Facebook for iOS where while the two-finger scrub gesture continued to activate the Back button, the button itself was not visible to VoiceOver. We immediately communicated this to the Facebook development team, and they have advised that their most recent build, 555.0.0., addresses the issue.
We appreciate the resolution, but also hope that Meta will strengthen quality assurance so that such #accessibility disruption is less frequent.