When a teacher gives a feedback comment to a student without setting a grade, the feedback comment is visible. But, it disappears from the students view in the assignment if another attempt is manually opened by the teacher.
It is still showing to the student in the Grades overview.
When the teacher grades the second attempt, the feedback comment on the first attempt is still invisible.
If the teacher goes back to the first attempt and grades that too, the feedback comment is showing in the assignment again.
(I have marked this as a 3.8.6 problem, since 3.8.3 is "overdue", but I am using 3.8.3 and maybe this has been resolved in 3.8.6)