I've seen similar issues to what you described but like yourself I can't figure out why or how - when I test myself it doesn't seem to register with the file.
It might be in your case that a user uploads a file - then deletes it but the form considers that a 'pass' when it comes to the required field.
One other option depending on your workflow could be to add a 'submission' flow where the user can save the record and come back and forth - but they can't submit until they attach and save a file. You could use form rules to lock / hide the submit. Something for consideration if you continue to see issues specific to file attachments.
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Chayce Duncan
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