I'm not that familiar with using woods other than mopani and Malaysian in an aquarium. Other knowledge is curing and working hardwood for projects or simply burning various woods.
If this is a tank fairly larger than your soaking container, it's possible that the higher water pressure is forcing gasses out tighter wood grains or knots (if any exist). A knot in the wood is a harder, tighter grain than the rest of the piece, of course.
If you see any stress from the fish, pull it out immediately. Do you know what type of tree it came from?