Hello Alessandro,
strangely enough I just found those customer-specific fields on the test tenant. The goal is that they are not visible by the development user there, but only visible by the normal business user, as if they would belong to some solution which is not maintained by the developer user who doesn't see them therefore. But there are no solutions on the test tenant, so they can't belong to any solution. This fact conters your statement that customer specific fields are tenant-bound and are not copied to the another tenant. What would you say about that?
Regards,
Leonid