There are several types of nosing profiles. That one looks like a slight eased edge and naturally these things can happen. The minimum I would suggest is a 1/8" round over. Example below, but these are solid treads.

He fabricated the treads with engineered? If so, I can see how that would splinter.

Does the floor manufacturer offer nosings? Are they available? It's not a sure things these days.

What he did takes work and looks great. I wonder if there's a remedy that will stop the splintering. I did a similar treatment at the insistence of a designer. After some initial splintering it seems to be holding up pretty well.

this is a modern mitered edge detail, gives the impression of solid treads....the execution..? 🤷‍♂️

Doesn't look like the installer applied enough glue if, any to the miter.