As above it could be to much washing ? , or to much water added to the grout ? , poor mixing ? but as you say it's a real shame as the tiling is very good, if its soft then I'd say its a weak mixed grout .
There are grout sealers that could potentially blend the colour more evenly but they will...
Little tip don't try doing it in one go apply a thin coat use two even three let it cure fully between coats, you may need to apply a primer between coats depending on the product used and especially over ufh rushing your prep effects the project finish.
Seen alot worse the chippage is minimal on the cuts its very difficult to avoid 100% the tiler needs to invest in quality tooling to improve on this grouting and the Silicon work could be better, new build properties are often tight on price the tiles are supplied by the developer ? so any...
I wouldn't do it that way any movement or flex in the sub floor will caused problems, I would be looking at a overlay system u heat do a range of products consultant them on your best option .
Very poor still I think this chancer has had his day it's time to move him on pay him 50% at most then get a decent tradesman to rectify his efforts. As a example this is how it should look and your floor layer is not gonna be happy 😤.
Wow is putting it mildly this chancer has ruined your careful prep work no other option but to remove the shoddy work and start again it should look more like this feel for you this is a harsh lesson to learn ☹️ . Regards Kop
I would say copy what you have posted if that what you really want bear in mind that you will have no access to the drainage once completed ?? a 25mm cut strip for the top edge , mitering hmmm bit of a risk on the old tootsies if you should slip mitred edges are quite Sharpe , trim it and water...
Not very professional but I have seen worse its a bit late to find fault now I would list down all the faults you are not happy with then talk to your contractor and come to a agreement on the best way forward, or kick his arse off and get a proper tiling professional in :p:p sorry that wasn't...