Taking into account the MDL-57957, a similar problem with not displaying the feedback file is repeated in version 3.9.

 
Typical situation:

• The assignment is NOT graded, only the task is marked as evaluated, eg by inserting a file with feedback (like PDF file). * Students who have successfully completed the assignment on the FIRST attempt see this file with feedback on that attempt (thats correct).
• However, if the teacher creates ANOTHER attempt for student to upload a new version, the previous feedback file will NO LONGER BE DISPLAYED on the previous attempt and the student will not find it.

So the problem is that the feedback file does not link to the given solution attempt.
The way this can be influenced is that the assignment graded (eg by points), then a file appears for each attempt.