Required upload field doesn't work in some cases

I made a form with a required upload field and users do not allowed to go to next step without upload the file.

In 95% of entries this works but in the other hand 5% of entries, useres are submitting the form without this file uploaded and that is a problem for my campaign managers.

If any entry doesn’t have this file is automatically rejected and this increase the number of request made by users that want participate in this campaign.

How can I resolve this issue?

This is the campaign form:

Hello. I recommend contacting support for this issue, for the quickest resolution:

Thank you.