Partition walls cracked horizontally

Hello, how can it happen that almost all non-load-bearing partition walls in my house suddenly have horizontal cracks from one day to the next? 11.5 cm and 17.5 cm HBL stones were used. I know that there are wallpapers that can cover these cracks, but I am worried about the bathrooms that are going to be tiled. The structural engineer says these cracks can occur due to the deflection of the ceilings and are harmless. Should I believe that?

Oh God the house is collapsing. That is typical, everything slapped on too early

The partition walls were plastered 12 weeks after the masonry.

The structural engineer is actually the professional, he should know. The crack itself can of course represent a visual defect, provided it has a certain width (there are limit values, I think over 0.2 or 0.3 mm). If you are building with [GU], he will probably say that the painter will fix it ;-)

and if painting is done by yourself, the construction company will also say so. Plaster cracks are well permanently covered by painter's fleece or wallpaper

and in the bathroom? It should be tiled up to the ceiling.