Why does modifying the “read more” link remove the link class?

I’m using the following code (found on the WordPress.org site) to modify my (more…) text:

The code works as expected, in the sense that (more…) is properly replaced by (Continue…), and the link works as intended. However, there is a peculiar issue: The class more-link has disappeared, which means styling doesn’t work. I attempted to circumnavigate the issue by adding styling inline (return '<a style="font-size:0.8em;" href="'%20.%20get_permalink()%20.%20'">(Continue…)</a>';) but that hasn’t worked either.

My Question:
Why has the class more-link disappeared, and how can I restore it?

Note: I add the code above to functions.php; I’m not using a child theme.