Two reasons why I don't think it is user error:
1. It is happening new this academic year (first year of Moodle with Atto since upgrading from 2.6 in Summer 2015) with at least 5 different teachers who have all used Moodle for 5+ years here with no similar problems previously then no I don't think every time this occurs (which seems pretty frequent from what I gather) it is the teacher has added feedback to the wrong student.
2. Also just tested this with a grading page showing the right student but wrong feedback, and refreshing the browser page shows the correct feedback.
Yes I'm aware of that issue (MDL-42306) but what that relates to is the wrong student being returned from what was expected. I have tested this and we can replicate that error but the feedback box matches the student being graded, even it it wasn't the student the teacher is expecting to grade.