has to submit CF7 form twice for signature to work

signature field in contact form 7, when submit, it is not sent in 1st ajax call, the fields in form post are empty
signature-364:
signature-364-attachment:
signature-364-inline
this happens starting from version “Version 4.2.1”
but it work well on “Version 4.2”
currently if form is submitted twice due to empty required field in the first form submission, the signature is sent in form post with the 2nd submit click.

I sent you above my debugging result for the problem, hope you get it fixed, thanks alot