Editor to Block migration - custom fields

Hi, I’m facing the migration from editor to block. Everything is ok but the custom fields which was defined in the old editor plugin. I can’t find a way to migrate them to the new component.

Example:
Old Editor plugin → type: image; additional custom field “css”
New Block component → type: image; additional custom field “css”.

After the migration (which means - entering the page from panel) the custom field doesn’t have any value.

What I am doing wrong?