Water Level Inconsistent

This basin used to be full all the way across to about the line where the cracking tiles are now. I built a bunch of tiles in the water out of really hot igneous rock (although some of the tiles might have been regular temperature rock as well). I come back a little later to find the left side of the basin almost completely empty, the water seemingly moved to the right side and dripping back down over the tiles. I was working just out of sight above on the tiles at the top of the screenshot, and I have no idea what happened. There used to be more tiles (some of them broke; I'm not sure why. Maybe from the heat?) but the tiles didn't go all the way to the bottom.