Unless I’m misreading your question, the custom length appears to override the attribute length. Doing some tests with both text box and text area, I wasn’t able to enter any text above the custom length when creating new objects:
However, when trying to edit existing objects, the text box custom length doesn’t appear to enforce the limit. But the text area widget does enforce it:
So, when creating a new object, both text box and text area work. But when editing existing objects, only the text area appears to enforce the custom length. It appears to be a limitation when the text box?
Hope this helps,