Encountered the same problem recently on that rubble pile too. Tried the vial on the nearby rubble pile (the one in front of the starting point) and found out what the issue is - the tiny smoke animation appears to be the part of the event, so it is counted as an unpassable tile. Simple solution - move that "rubble" event down one tile. In case Miro would like to make smoke animation passable, I'd suggest him to make it a separate, "move through tiles" event.