If a teacher adds a feedback to an assignment submission is stored and visible in the grading overview for the assignment. If the teacher comes back to the grading area the old entry isn't anymore more visible and he has to reedit it.

It should be possible to correct or cahnge the feedback text after first saving of the changes.
Some time ago the journal module entries were moved to assignment and journal was taken from the core. Lots of teachers are using the assignment module for continous feedback in a process to a student. Therefor its required that old entries in feedback field can be reedit or new text can be added.