Saving a form with an entity pending creation skips validation

I have an entity that contains an Inline Entity Form. I need to validate a "deadline" date field in the inline entity to ensure it isn't past an "overall deadline" field in the parent entity.

Using the "Add New Entity" button opens an inline entity creation form, as expected. However, saving the parent form without saving or cancelling the inline creation form causes the inline entity to be automatically created, bypassing parent form validation and thus permitting erroneous field values.

Expected Result:
When the parent form is saved, if an inline entity is pending creation, but hasn't been saved or cancelled by the user, it should either automatically cancel creation or throw a form error to allow the user to decide whether to create or cancel the pending inline entity.

*This is my first time reporting a bug, so let me know if I'm missing any required information in my report.