when i was building a map, and started placing down dirt using command blocks (/execute on a 20hz redstone clock). when i placed grass next to it, nothing happened. i had /gamerule randomtickspeed set to 100000, so normally everything should be covered in grass immediately. i tested some things:

when dirt is placed by a player using /fill or /execute x y z fill, grass does grow on it.

when dirt is placed by command blocks using /fill or /execute x y z fill, grass doesn't grow on it.

when randomtickspeed is set back to 3, it still doesnt grow on the dirt.

i hope this is usefull, and can be fixed soon,
Harm Prins