The cause was calling focus/setSelectionRange on a textfield within a click handler. When I commented these out then the problem disappeared.
Not a rare combination and I’d have thought beta-testing should have flushed this out, but there you go.
Edit:
If you want to try reproducing this, go to https://cycle.travel/map/mobile?debug=1 , and click within the 'From' or 'To' field. It crashes every time on my iPad running iOS 14.5.