Normally, the Mesa Plateau F biome generates trees. It seems to not be doing this, but it still generates dirt and grass. The seed I used to find the Mesa biome was "Mesa?" Similar (possibly related) issues are occurring in other biomes as well.

Confirmed Cases:
Oak Trees in Mesa Plateau F
Spruce Trees in Extreme Hills+
Swamp Oak Trees in Swampland
Giant Mushrooms in Roofed Forest

Unconfirmed Cases:
Pine-Shaped Spruce Trees in Taiga
Oak Trees in Forest (too short?)
Giant Mushrooms in Mushroom Islands
Oak and Spruce Trees in Ice Plains