I think I've found a stupid mistake in my extended directory code that has been adding the wrong feed URL for some blogs to my dynamically generated OPML file. Hopefully, I've sorted the problem but will need to wait for some webmentions to come in from different blogs to check.
It has meant doing a bunch of find and replace in my commentmeta table to correct those wrongly entered.
If you had previously grabbed the OPML file from the /directory page then I apologise. It should now be okay and, with any luck, stay that way.