Seems like a service that promotes an attention to detail but didn't notice its error in posting to an unrelated forum.

I wouldn't agree that "they cut corners". This is a pretty normal method of treating external corners. It's cheaper than adding a panel but it certainly isn't the best way. I suggest you tell the developer you're not happy with the fitting of this because it will keep moving out of line and possibly falling out completely. Tell them you would prefer a finishing panel to the floor. I think the polite and nice approach is more likely to succeed than threatening to report them to the NHBC

Could you make a feature with that end section and but the wooden strips diagonally? I've seen a photo of these been done but I cannot find it anywhere. You could then maybe use that are your feature area with a lovely floor standing mirror and a big plant or some other things you want to stand out in that corner. I think you can make an amazing feature from your mistake.... be not disheartened!!!

Can you do a full measured plan of the whole room, marking on doors/windows. The drawing indicates there could be living space/garden views behind the stools, yet if sitting on a stool you would have your back to what could be a nicer view of the entertainment space, if that's what it is.