The bug

All biomes where large, branched oak trees would naturally occur are completely devoid of them with the new world generator as of snapshot 18w06a. It's also not possible to grow them with saplings anymore.

Notes

• There was a similar, but not identical issue in the past regarding large oaks not generating in forest biomes, see MC-29844. They could still appear in other places or be grown from saplings.
• Tree stumps appearing all over forests and jungles as of snapshot 18w06a are caused by the "balloon oak" variant generating upside down, see MC-124897.