It was done by hand. You can still see his pencil lines. Looks like he wiped it before the catalyst kicked and the adhesive shrunk below flush. It's nasty.

thank you for your feedback!

what do you recommend to solve the problem?

here is one more photo so you can see the location, right by the window so the seam is very visible in the light

It's a do-over. Those white French miters have to be done on a CNC machine if you want them to be inconspicuous:

agree, joint is poor. Looks like they had no idea how to do a Mason's Mitre