I found the source of the problem.
It’s a change to this file, and will impact some themes.
the-events-calendar/src/views/month/nav.php
At line 19:
<h3 class="screen-reader-text" tabindex="0"><?php esc_html_e( 'Calendar Month Navigation', 'the-events-calendar' ) ?></h3>
The problem is that the CSS in my theme apparently doesn’t have an appropriate specification for the screen-reader-text class — the problem disappears when I switch to TwentyFifteen.
You’ll find information about the required coding for that class here:
Hiding text for screen readers with WordPress Core
I resolved the problem on my installation by adding the required CSS specification to my child theme style.css file
However, I’d recommend that you also add that class to the css for your plugin, at least for pages generated by that template. I am using the Evolve theme (free version) – but I’d assume that other themes might have issues with this class specification.