Sorry – yes it has!
Adding
add_action('pre_get_posts', 'my_pre_get_posts');
fixed it.
How embarrassing.
Noticed I had forgotten to add
add_action('pre_get_posts', 'my_pre_get_posts');
But that hasn’t helped either
Hmm
Just installed a child theme on the default 2021 theme and still not alphabetically sorting.
So appears not to be my bespoke child theme.