This has not been tested and I don’t know if it will work.
Use an acf/pre_save_post filter https://www.advancedcustomfields.com/resources/acf-pre_save_post/
You’ll need to implement some checking to make sure you’re dealing with the right form, and you’ll also need to look in $_POST['acf']
to get the values.
Once you’re done processing the form
unset($_POST['acf']);
ACF only begins the save process based on this having a value…. I think.