This is because the topmost staff is being used to draw the markers and in this case there are only multi-measure rest objects there in the first five bars. A worse case is if you create a click staff at the top - this is hidden when the cursor isn't on it, and when hidden the start/end markers are not shown.
This does provide a workaround - create a click staff and whenever you want to set/see the start/end markers set them on a click in the click staff (you would mute the staff for playback of course).