re: I have made no changes to the template or page since Jan 2016. What could be causing this problem and how do I fix it?
I’ve seen this before… the cause, generally always the theme. The theme dev for some reason decides to globally change the HTML span element from an inline element to a block level element. In you case, it is something different…
Navigate to the Connections : Settings admin page and click the Advanced tab. Scroll down to the bottom and enable the Template option in the Compatibility section. This fixes instance where the theme or another plugin improperly make changes to the core WordPress wpautop filter.
Hope this helps!