Required validation not working on post_content field

I have a post_content field in a frontend form. The field is set to required using validation like the following:

'validation' => array(
    'rules'  => array(
        'post_title' => array(
            'required'   => true,
        ),
        'post_content' => array(
            'required'   => true,
        ),
    ),
),

If I try to submit the form with text in the post_content field I get the "Please correct the errors highlighted below and try again." message at the top of the form and the submit button becomes disabled.

If I try to submit the form with the post_content field blank, validation works properly... I get the "Please correct the errors highlighted below and try again." message at the top of the form and the "This field is required." validation message below the field. The submit button is not disabled and I am able to add text to the post_content field and click submit again. However, the form does not submit. I still get the "Please correct the errors highlighted below and try again." and the submit button is disabled. (The validation message below the field is now gone).

So it seems the validation is not completely working correctly as it always fails even if the field contains content.