Salesforce concept : Validation rule does not fired after approval process field update

Validation rule will not be fired by Workflow rule or Approval Process field update based on document.

Both API level and UI level change of record honored validation rule , but field update is an exception.

Which means , if you have an validation rule block field_ABC value to be "ABC", if you used a field update to do this, the value of  field_ABC can be set as "ABC".

Official IdeaExchange 1

"because updates to records based on workflow rules doesn't trigger validation rules,workflow rules can invalidate previously valid fields "

Official IdeaExchange 2