Aloha Elliot,
I looked up the docs and found that the problem is caused by a subtle difference between update_post_meta() and update_metadata(). In version 5.9.2, you switched to using update_{$type}_meta() in includes/acf-meta-functions.php, but these wrapper functions do not allow updating meta on revisions. See the comment in the core source:
https://developer.wordpress.org/reference/functions/update_post_meta/#source
This also applies to delete_post_meta(), FYI:
https://developer.wordpress.org/reference/functions/delete_post_meta/#source
So basically when you switched, all acf_update_metadata() and acf_delete_metadata() calls now operate on the parent post if a revision ID is specified. I would suggest reverting to update_metadata() and delete_metadata() to avoid this.
https://developer.wordpress.org/reference/functions/update_post_meta/
https://developer.wordpress.org/reference/functions/update_metadata/
