I have a problem with a layout field nested into a structure data field

I am working on a system to manage different banners (composed using the block layout system) on different pages (each banner can be associated with one or more pages).
I created a structure data field having 2 fields inside it, a block layout to build the banner, and a page selector to indicate where it should be used.

It works.

So I added to the block layout the ability to indicate settings (specifically a custom_css field to customize the banner).

What happens at this point is that if I value the settings once the page is saved, on the first load the block layout field automatically empties, deleting itself.

Can you tell me why or what can I do to fix this problem?

Could you please provide some code to reproduce this? Can’t quite follow what is happening where.