I had the same problem. I fixed it by using the ep_enqueue_script aproach shown here: https://codex.wordpress.org/Quicktags_API listed under “A more modern example”.
It’s the same thing that Tom mentions. Instead of using ‘admin_print_fotter_scripts’ to write an inline script on the page, the example shows putting the Qtags javascript in a separate JS file, and loading it with ‘admin_enqueue_scripts’ as the action and calling wp_enqueue_script.
The Quicktags_API link above shows both methods, but the inline script method stoped working for me.