Using Alien Biomes in my world and noticed that Arcanyx doesn't have any trees as it should from looking at the images, but it turns out they for some reason weren't spawning if I used Alien Biomes, as I turned it off and checked again and the trees existed

Alien Biomes prevents autoplacing of all tree prototypes except those on a hardcoded list called trees_to_not_disable, which the Arcanyx trees of course are not on. There's nothing I can do about that. This issue will have to be fixed by Alien Biomes.