Validation messages for Package custom coverage field are not shown

Steps to reproduce:
1) Edit a package record
2) Go to custom coverage
3) Enter a wrong date format (e.g. 1 or 01/5) in the End date field
4) Save button is active OR sometimes it is not

BUT no error validation displays

If the Save button is active, you are able to save without any error validation
Expected outcome Error validation should display inline and Save button should be disabled.

Note: Title+Package does not seem to have the same issue.