Okay, my temporary solution would be the following to anybody experiencing problems, based on a combination of advice from everybody.
1.) Disable the Yoast XML Sitemaps feature and install another XML plugin, for now at least. It appears to be somewhere within this that is causing the problems.
2.) Uncheck ‘Enforce a trailing slash’, and possibly ‘strip category base’ too. I am yet to fully discover whether the latter causes problems, but the first definitely does. To be on the safe side, I am using an external category base stripper until it is confirmed as working, as I don’t want to risk it. I’ll keep you updated when I do try it, though.
Thanks for the help everyone, and good luck!
UPDATE:
It seems to be working with ‘Strip Category Base’ checked, ‘Enforce a trailing slash’ unchecked, and Google XML Sitemaps installed.
Could somebody visit https://ryancaseybooks.com, have a click around the categories, and verify this for me?
Regards,
Ryan.