&nbsp being inserted between a and href on post publish

How are you adding this hyperlink in the editor? Is this happening on any other pages?

Default editor. Usually adding via the insert link button but have also typed them in manually. This isn’t the only page it’s happening on. Here’s a post from earlier today with the same problem: https://travelinlibrarian.info/2015/05/uc-berkeley-project-discovers-writing-from-mark-twains-early-days-as-newspaperman/

Also, just changed to a new theme and the problem hasn’t gone away so it doesn’t seem to be a theme-based problem.

Try disabling all of your plugins temporarily to see if the issue goes away. It’s definitely a widespread issue – if you look at the source code of the page you can see all sorts of data is being displayed incorrectly.