while this may seem like a bug, it actually is not a bug. Unfortunately it is not possible for us to have spawn colliders on edges like shown on your provided screenshot.
That is because of the given restrictions we have in the Mapping Editor, so to summarize the situation, whenever there is an edge south of an area which players can access you wont be able to encounter pokemon.
We try to avoid these cases as much as possible but it is not always possible with the way we are creating maps.
I hope I was able to clear your doubts and explain the situation to you in an understanding way.
Clumsyclef's post in Seafoam B4F Non-encounter line was marked as the answer