This wasn't a browser issue - the issue was that the booking was for a round trip, but only the outward portion was canceled (at that time). The chat hotline could nevertheless change the return flight at the same time, but the automated rebooking on the website couldn't.

If anyone is still playing these games (speculative bookings on cheap flights likely to be canceled), note that rebooking rules became more restrictive. In case of a cancellation, you can still rebook on the original route to any date, but when rebooking to a different destination, the new flight must be within 2 weeks of the original flight date.