Thanks a lot for your time and for the video!
Actually this is correct behaviour! It is on purpose that the longest layer does not have the button, because basically it does not technically make sense to be able to change what is beyond the last frame. Since the scene ends at that point anyway.
However! I must admit reading your post, that this might be confusing. Maybe it is better to have the button to avoid this seemingly weird behaviour. Also when the layer is not the longest anymore, you could have set the button beforehand if it existed.
On the todolist for careful consideration! Thank you!