That glitch only happened for me once in Rivertown (and was gone after I closed the map and reopened it), and not at all in the dark forest.
The above ground, grassy part of the easter egg area is in the upper section of the map, along with the rest of the outside areas of the dark forest. You can see the easter egg area in the north west section of the dark forest, across a relatively narrow section of mountain. The underground areas of each region are all in the lower part of the map (as the game sees it), grouped at least partly on their shape (to fit together more efficiently).
