I have two diagnosis line item for a particular repair order, and three custom fields

One checkbox , on clicking the checkbox the second field(date field) gets mandatory. Only after selecting a date in the second field I can clear the error.

My problem here is , if I enter a date in first line item, save it , enter into second line item, check the checkbox, leave it as error and save the repair order, Then when I go again into the second line item, I get the value of first line item's date field value automatically populated in second line item's date field .