A few people have this same problem so it looks like a bug. Selecting a midi folder track and a single midi track and attempting to edit their notes in Key Edit causes the notes of the midi folder track instruments to disappear from Key Edit.
Could you please link to the other forum, where it was discussed? Or how do you know, other people have the same issue?
I think this was already discussed here on the forum, and reported to Steinberg.
I can’t find an option to delete a post.