Why do validation rules not work if there is a value defaulted in the field?

We have used the following expression to default the effective start date on the Request Position Change to be the current date:

[[ (new Date(new Date().getTime())).toISOString() ]]

We are now trying to add a Field Validation rule to throw an error message if the user attempts to change the effective start date (since we cannot make this field read only). Here is the Condition and Messages configuration:

However, the error message is prompting as soon as the end users goes to the Request a Position Change page, even if they do not touch the date field. Why is the validation rule not working as expected? We have set up similar rules on different pages that do not have a defaulted value in them, but the rules are working as expected.