I was making some changes to the way comments work - specifically with regards to required fields - and think It might have had a knock on effect with webmentions. This post will serve as a test.
In order to align native comments with webmentions I wanted to remove the need for a commenter to enter an email address but still have 'Name' as a required field. Unfortunately, the setting in WordPress is for both or neither.
By hooking into
preprocess_comment I could prevent a comment being posted if
$fields[comment_author] was blank but, unfortunately, this seems to stop the processing of webmentions - presumably because at the point of processing the author field hasn't yet been filled from the webmention data - so have had to revert the change.