I was playing Pokemon Showdown, and was giving my Gastrodon a moveset. My sister came along, asked what the arrow was, and I told her it was to change it's color. I clicked it and said I should use the green one (East Gastrodon). I used it in battle, but after that I realized it was illegal... yet still valid?
The game might’ve gotten confused by the two, Pokemon showdown tends to get forms incorrect. This has happened to me before, it’s fine. Just occasional bugs that get into the game.
My Gastrodons can learn rock slide perfectly fine. Maybe this glitch was fixed.