Hmm. It seems that the carnivous tree battle is tied with the gfx tile.
I would have assumed that only the first two tiles are the blocking ones that trigger the battle... But it seems that all of the tiles trigger the battle.
EDIT:
Related article.
Btw... Since you have the debug mode on, how about going to the large, inaccessble part of Dezo near Esper Mansion to see if there's any battle encounters?