Access denied error when creating new document set in a SPO document library with unique permissions

The escenario is:
A document library in a team site with unique permisions, and with document set content type enabled to be created. We break the permissions inheritance for this doc library in order to grant edit permissions to a non member user from inside the Organization.

We don't want this user to access to any other resource of this Team Site, only to this document library.

This user can access to this doc library using the direct link to this Document Library, and create/edit any document (as expected), except from document sets content types. When trying to create a document set, SharePoint shows an access denied error saying that this user (with edit permissions given directly to this document library) has not enough permissions.

This user should be able to create and edit any content type enabled in this doc library, including document sets, as he/she has the correct permisions directly asigned (edit).

We suspect that this issue is due to the fact that the user only has edit permissions to the document library, but not to the "form page" or "welcome page" of the document set (that displays when he tries to create a new one), which seems to be located in other resource of the site (in which this user does not have any permissions).

Is this a bug? Is there any work around? Where are this "welcome page" of the document set located? Can we give direct permissions to this welcome page?

Thanks all!