plot dissappears when using bone meal on jungle tree saplings

While planting Jungle trees in a town (using towny)
plant 4 saplings, 2x2
use bone meal, the tree grows
then the entire plot disappears (maybe a 13x13x13 cube?)
I sink to the bottom of this empty cube
I can see up to the sky, but i start suffocating
use a pick axe to dig a hole around me (took snapshot, attached) so I can breathe
quit, rejoin
I'm in the hole underground that I just dug and I have to dig my way back to the surface.
This happened 3 times in about 15 minutes, then I stopped trying to plant trees.