Hello again,
I have added debug prints into the latest version 2.1.1 of the extension.
1. First enable debugging. Then, view a custom post. Either the UI will be shown, or you will get an error message in the debug.log file, explaining why the UI was not shown.
2. Let me know if you got a chance to test again with CMB2 disabled.
3. This could also be an issue related to the theme. Let me know if you tested with a different theme.
with regards