I am not sure what the best way to do that was and whether or not it could look like a clean fix. I would still like to explore what that fix might look like which would help me understand why it is a crappy solution.
It would be something like adding a corner of slanted tile there and grouting it into place. There is no way it will ever look like it was supposed to be there. And if you don’t get it sealed 100%, now you have a void which is basically permanently wet inside.
Another option would be to build up some silicone in the area, that is probably going to look even less good, but it will be easier, more likely to seal well, and can be removed. Do note if the tile is bumpy, getting the silicone out of the bumps later might prove difficult, but if the tile is smooth, you can scrape it off pretty easily. After you put it down, you can smooth the silicone out with a wetted finger or smoothing tool.
I would personally be tempted to go the silicone route if a full fix is off the table since mold is evil.