Lime-gypsum plaster crumbly and therefore defective?

I think the plaster in the photos looks normal.

Thanks for your feedback!
: maybe I really expect too much from a cleaner

Have you also tried at another spot, meaning where there is just plain masonry behind it?

Have you also tried at another spot, where there is pure masonry behind?

Not yet, I will try that tomorrow and also hammer in a nail. However, I think the "consistency," the crumbling, does not depend on the covering behind. If you slightly enlarge the cutout for the sockets, it continues to crumble freely. The lime-cement plaster in the bathroom tends to come off in "whole pieces" when tapped and requires more force to break/hammer it. But as I said, I don't know how it should be with a lime-gypsum plaster.

We are also getting a plaster like that, but it will take a while until it's ready for us. I think a plasterer (stucco worker) can help you best. From my layman's understanding, I would say that the plaster simply has a higher lime content. Do you have more details about the plaster used (e.g. picture of the bag)?

The plasterer himself will probably say it's normal.. Unfortunately, I don't know who else I could ask on site. My thought was also whether it has to do with the lime content. Maybe I'll just call a plastering company tomorrow and get some advice...