We observed dateField saveInto code executed even though user enters invalid Date manually. We see OOTB validation error message on screen but save logic getting executed successfully.
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The problem is with your rule!GBL_isBlank. It is not getting excecuted properly. Please check that rule.
I used GBL_isBlank (custom rule to check for blank and null values) to demonstrate the behavior. We are confident about this rule and its working as expected.
The issue that I'm reporting here is why saveInto is getting executed in case of invalid date is entered. The validation message that you see is OOTB not custom validation.
Just to be clear see below screenshot where the date entered in not valid (OOTB validation error is shown on screen) but saveInto logic executed successfully.