Further experimentation led him to turning off all the real-time effects he has applied to all 11 tracks in a multi-track project. And lo and behold this cured the dropouts on recording the 12th track.
I have added a note to the FAQ in the 3.5.0 alpha manual:
It may be worth adding on not on the Support page for real-time effects to the effect that they may (but only may) cause recording dropouts. https://support.audacityteam.org/audio-editing/using-realtime-effects
This emerged after I have been working offline with a user who was experiencing dropouts - he tried all the suggestions in the current FAQ: https://manual.audacityteam.org/man/faq_recording_troubleshooting.html#skips but to no avail.
Further experimentation led him to turning off all the real-time effects he has applied to all 11 tracks in a multi-track project. And lo and behold this cured the dropouts on recording the 12th track.
I have added a note to the FAQ in the 3.5.0 alpha manual: