Now here’s an question!
What did you all do before the colour matched Silicon was available?
You see in my opinion the builder did what we used to do - grout the corner and go over the dry grout with a clear or white Silicon.
The bigger issue here is why is it cracking so much that it’s become a major concern?
As a tiler, I Silicon the corner, fix the tiles, grout, then Silicon the tile face again. Always have always will!
Don't be so quick to judge the builder. Silicon is great unless you happen to have RIW tilesafe membrane behind. I would normally leave the internal corner joint free of grout then Silicon knowing the Silicon would bite nicely into the gap however CCL Wetrooms advise if you use Silicon then warranty will be void. Builder may have been following instructions. RIW also advise the same. And that's why I won't use the bitumen style membrane