Interesting. I see that rpg_core.js has defined it as an additional property of arrays:
Alright, well, maybe it'll continue to work then.
I'm still not sure why includes didn't work though. Since it is a default property of arrays, and that is what should be on the items, and more importantly, why it works fine in my test project, and at least one other project out in the wild.
But if this modification works for you, I guess we can leave it at that, lol.