Horizontal crack in the interior plaster of the new building

Sorry, but what are you talking about? Did you even look at the picture?

Yep, I did. These are perfectly normal settlement cracks/shrinkage cracks. They still appear after years. I know this from hundreds of houses and unfortunately also from my own...

why shouldn't cracks be filled with acrylic?

Because neither acrylic nor expanding foam are a cure-all – except for amateur home handymen. There are appropriate areas of application, and settlement cracks are not among them. You can’t even get the acrylic properly into the crack to fill it. If you want to do it properly, you enlarge the crack (chisel it out), then fill it properly with filler, sand it, and then paint it. The acrylic bump you create will always be visible and will annoy you.

Acrylic is recommended for settlement cracks, it is also stated on the products. You can fill the cracks with it and it looks good visually.

I agree with you on two things: waiting until everything has settled a bit and secondly, your approach of scraping out the crack and replastering/spackling would be the best. But also very time-consuming.