If I remember correctly: The shape of the boundary between Riverfront Museum's main building https://www.openstreetmap.org/way/283670678 and its canopy https://www.openstreetmap.org/way/662295794 can't be reshaped to match aerial photos, because they're both part of PRM's building relation https://www.openstreetmap.org/relation/9205982 and possibly also because way/283670678 is coterminous with part of the boundary for the pedestrian plaza https://www.openstreetmap.org/relation/9206026 - This is a specific quirk in the iD Editor in which iD decides that it's creating an overlap when it's really a shared boundary, or sometimes there really is a momentary overlap, so iD just reverts the move as soon as you make it, without waiting to let you adjust anything else to fix it instead.  There's even a possibility that the present shape may already be out of specifications for iD, so any movement at all will be still unsatisfactory to iD.