Problems with validation if field is not mandatory

[sg-splus]

I have a form with a field that should only be filled with numbers but is not mandatory. I set validation to "numbers only". The result is everything I didn't expect. The user can fill in anything they want and submit the form but if what they submitted is not numbers only it won't show up in the mail to customer service.

Please fix this.

I use TYPO3 11 with powermail 10.7.3