I try to reproduce the issue on the demo site but don't see that. I can set a date, save post and delete it as well. You can try to deactivate all plugins except Meta Box, MB extension plugin, switch to a standard theme of WordPress and check the issue again.
I switched the theme to twentytwentyfour, deactivated all plugins except Meta Box and installed the code snippets plugin in order to run my fields. The issue still persists