Shower Niche Cracking
I had a complete bathroom reno done ~6 months ago that included a tiled shower installed with the schluter system and a tiled niche. One of the bottom tiles in the niche cracked about a month after install and the contractor's tiler came out and removed the middle two tiles to replace the cracked one (picture 3). Since then all four tiles have cracked in a similar fashion to the first one, leaving the niche in the state shown in pictures 1 and 2.
Can anyone lend some insight into why the tiles are cracking? I have the contractor coming out in a few days with the tiler and the manager of tile shop to take a look. He's proposed potentially putting in a piece of quartz to replace the bottom shelf, which seems like a reasonable solution to me but I don't know what I don't know. Does this seem like a reasonable way to remedy the situation? What questions should I be asking? Thanks in advance for any and all info and advice.