If there is some kind of river or ocean around a roofed forest you might find a dark oak tree that spawned (naturally) over sand or gravel. In some cases they spawn in the air. Fix it please 😛.
Seed: -943123844013429995
Picture 1 Coords: x: -1565 y:62 z:1477
Picture 2 Coords: x: -1685 y: 62 z: 1619
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Please provide seed and coordinates.

Confirmed, but all Dark Oaks are spawning in the Roofed Forest biome.
This are "features" of the world generator, see MC-610