I would look at the logs for the date specified and see what the student did. I suspect that they did not click the submit button or something. Submissions do not just disappear - they were either never uploaded (most likely-students lie all the time) or it was uploaded and then deleted. I suspect the student started to upload, got distracted, didn't complete the process and is blaming moodle.
Assignment
Dissapearing Assignment Submisions
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