First of all, I must apologize to the local community for my (now deleted by me) "quick shot" earlier regarding the loading time of the photos!
After all, one is only human ...
Addressed to the questioner:
What we see here is a screed stopper strip made of a closed-cell foam, common in screed installation to ensure the edge joints at rising building components.
In your case, the stopper strip, here at the shown spot probably used to form a movement joint, was worked on during the smoothing of the screed surface. This can happen, it happens a thousand times on all construction sites and is not a big deal.
The floor layer must perform a mandatory cleaning sanding before continuing work anyway. Since the cement layer above the visible stopper strip is only extremely thin, it will inevitably be removed during the cleaning sanding. It can happen, it is nothing serious!
What is not okay, no matter what some shouted as an interjection, is the rounding in one of the corners. The screed standard clearly states that the stopper strip must be flush. Also in the corners. If the stopper strip had been cut in the corner before the screed was installed, then the adjoining (continuing) piece set at a right angle and the stopper strips secured against slipping, there would have been a sharp corner formation there as well.
Just lay tiles over it, please do not take this answer seriously! The floor layer would have to raise concerns about the execution of the screed at this spot (or spots).
That means: Written (!!) complaint of defects by the builder to the screed installer so that he can rework it.
Usually, the additional effort associated with the reworking, if the floor layer should do it (which usually will be the case), is deducted from the screed installer’s invoice.
I am not a lawyer, but this procedure is practiced at all construction sites and is also legally fine.
Means: Written complaint of defects to the screed installer with the heading "WRITTEN COMPLAINT" and demand with a deadline to eliminate the improper corner formation(s)!
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I hope to have made up for my quick shot earlier with these more extensive explanations.
A nighttime greeting to all: KlaRa