Out of nowhere, the water level in the bowl in one of our toilets is
filling to about 3-4 inches below where it used to. It flushes
normally and the innards seem to be working fine (though the flap is
dropping back down pretty fast, perhaps faster than it used to), but
the level of the clean water in the bowl after flushing is noticeably
less than before. Not a huge problem, but I'm left wonder why. Any
ideas?

Mike