The input text field validate in sfdc flow behaves opposite of the generic sfdc validation rule.
Here the validation formula is evaluating whether the input is valid (true) or not (false).
Formula: LEN({!Comment__c}) <= 255
The formula expression evaluates to true for valid input, and evaluates to false for invalid input.
It displays error message to the user for invalid input only.
Alert: Maximum characters limit is 255
