This behaviour is correct. If there is a personal note for a person for this lesson, then obviously they at least were a member of the group, and maybe left it later. Why should personal notes be discarded in that case?
As in, a lesson is held in several classes (like WPU), but a register printout is created for only one of the groups? In that case, you only want to show the personal notes concerning members of this one group?
Idea: When a personal note is created, it gets a second relation to the group it is related to, so we can later track it without relying on the person alone.
One thing I recognized just now: Persons, who where a member of the group sometime ago, also wont be displayed in the other pages (persons list, persons detail pages etc). How we deal with this behavior?