Yes, there's a metal flange around the drain pipe. 3/4s of the flange
is resting on the ceramic tile, but the rest is unsupported w/ no tile
or subfloor underneath it. To clarify, the gap is around the drainpipe
and the flange. It would be difficult to replace sections of the
subfloor b/c it consists of 2 layers of wide, long planks that extend
into the next room. I was thinking about using epoxy or something to
patch the subfloor, and then replacing a section of the cement board.