Similar to the gap that can exist between your wall tile and tub you may have a space between the floor and your bathtub.
Transition from wall tile to bathtub lip.
Was the shower water seeped to the lip area from the bottom caulk and then travelled laterally out and wet the dry wall adjacent to the tub.
Above the lip of the tub.
I cover some of the specifics of how to layout and set up a tub to install tile around it.
The tub is designed to sit on the floor and be screwed to the studs kohler recommends setting in mortar which i did.
Then apply a generous amount of thin set to the bottom bathtub tile and position it 1 8 in.
This kohler acrylic tub is thick the flange is more than a 1 2 thick if you run the cement board over the flange and the tile which is about 1 2 thick when thinsetted it will come too far into the tub.
This video will show you how to set tile around a tub.
This issue is sitting there for a long time now and really bother me.
This gap may widen over time as your home shifts.
The best approach when installing cement board or other tile backer around a tub is to keep the cement board out of the lip area.