How do I restore the “Read More” functionality?

When writing a long post in the classic editor, the “Read More” button doesn’t function correctly. Instead of adding the “read more” tag where the cursor is located, the “read more” tag is added at the very bottom of the post.

I have been able to manually add <!-- more--> within the text tab, but I’d rather not require my students to do the same thing with their own posts.

It occurred to me that the problem might be caused by the Tiny MCE Advanced plugin, but this happens whether or not Tiny MCE is activated. Also managed to replicate this problem across different themes.

Thank you to everyone who has worked on this useful plug-in.