This issue looks like
MDL-58697 but the cause is different.
A student submission is displayed with the status 'No submission' but there are submitted files.
Steps to recreate:
1. Teacher creates assignment with following settings:
Online text - checked
File submissions - checked
Word limit - 100
2. Student clicks 'Add submission'.
3. Student uploads a file.
4. Student types more than 100 words into Online text field.
5. Student ignores warning and clicks 'Save changes'.
Here is a screencast made by my brilliant colleague, Olivia Fox, who got to the bottom of this mystery: