TL;DR: The Helioptile spawn “bug” is very likely due to OSM tags.
I took a ferry this morning that travels a route that has a narrow section between two islands. In this section, normal event spawns occurred.
In addition to Helioptile, the spawn pool includes:
– Avalugg
– Samurott
– Typlosion
– Braviary
– Regionals (Lunala, Throh, Shellos, Panpour, Seviper, Illumise, Tauros, Durant were seen in my case)
The attached screenshots show before that section, during that section (including how close the islands were), after that section, and a rough sketch of where that section was. My OSM skills aren’t amazing, so I will leave it to the folks here to see if they can figure out the tags (or lack thereof).
Credit to this comment for sparking this this: https://www.reddit.com/r/TheSilphRoad/comments/1mf5efn/comment/n6f6atk
by Pendergirl4