This is like a note as it is not so harmful now.
Click runway to open the dialog. The posbutton is momentarily displayed at the bottom of the dialog and disappears.
I don't know if this is supposed to be displayed for some purpose or is mixed incorrectly.
EDIT:
This looks like a posbutton for the reserved air convoy, but such a momentary display is not visible on the track dialog. And the pos button of convoy reserved on the track is not displayed. It seems that the bug is that the posbutton is not displayed on the railway track dialog.
This bug may have been fixed by updating the dialog.
I can no longer reproduce this - marking as closed.