Dynamic trees version : 1.12.2-0.9.5
Forge version : 14.23.5.2838

Superflat preset : 3;minecraft:bedrock,44minecraft:stone,2minecraft:dirt,minecraft:grass;1;mineshaft(chance=1),decoration,dungeon

I think this has to do with the fix you implemented in version 0.7.4 :

Bugs Fixed

Workaround for Better Foliage bug. Thanks mangoose3039
Fixed bug where species growth functionality wasn't being respected.
Fix for issue #36(Compatibility with Better With Mods "Hardcore Lumber")
Fixed world generation bug when there's no ground
Fixed client crash with rooty dirt block color multiplier
Fix crash involving TNT
Fixed problem with trees not growing in multiplayer.
Trees no longer generate in superflat world.