We have lots of modal forms in Workplace and hit it occasionally but it can also be reproduced in core, for example:

1. Go to Site administration>Users>Privacy and policies>Data registry
2. Press Edit and then Purposes
3. Press "+"
4. Open help of any element
5. Press "Esc" on the keyboard
6. Modal dialogue closes but the help text floats on the page and is impossible to get rid of it

Some related articles from the internet:
https://stackoverflow.com/questions/42557799/hide-bootstrap-popover-if-parent-element-is-removed
https://sandbox.moodledemo.net/