I wouldn't be surprised if this technically isn't a bug, but it still seems like a design feature. This is probably rare, as I haven't ever seen it before, but sometimes Highlands islands can generate with almost no trees. My assumption is this has do something with the procedural generation just causing this rare chance where there's barely any trees from an extremely low tree density, however I do not know much about how the procedural generation system works for this stuff. Note that this is in a Highlands biome so its tree generation is still enabled/functional, as I have seen on other islands.

Caused by Orbis, won't be relevant in port.

Tree density also seems too low in general on the Aether's islands.