The problem is that for the user it is very hard to comprehend which fields he left not filled.
Please visit our site and try to click User Registration (desktop. Chrome). Then click Register. Site will scroll to the first missing required field (on desktop). Please note that the field is hidden under menu.
Now try it with Safari on iPhone and fill in all the info and skip for example file upload, it will be still unclear that user forget to upload file.
I would suggest to mark very clearly what fields were not filled for example same way it is made in WPForms plugin.
Thank you