There’s a problem (at least in the iOS app) with multi-artwork spots shown as red dots on the map.
It works correctly the first time you tap on a red dot—you get the circular artwork thumbnails for all the artworks at that location. However, after opening one of the artworks and then returning to the map, tapping the same red dot again does not respond. To make it work again, you first have to tap on another (single) artwork somewhere else on the map, close it, and only then will the red dot respond again.