In 18w20b, trees are generating in the ground or not at all. The biome is shown in the screenshot. Affects forest, swamp, roofed forest, taiga, jungle, etc. Either it does not affect extreme hills with trees, or trees randomly decide to generate. But when I am flying by trees that generate there is extreme generation lag. This also appears to affect underwater plants like kelp and sea grass.
The specific seed is: -6400733238810633415