Hi Beth,
With related sites mode user and roles and site security settings are shared across sites so users in a given role are in that role for all sites and in general have the permissions of that role in all sites. Therefore currently roles that can upload can upload in any site.
The purpose of the Site Editors is to be able to make someone basically the equivalent of Content Administrator except limited to a specific site. Whereas memebers of Content Administratos in related sites mode can edit anything in any site and do not need explicit permission granted to that role. We filtered out the Administrators and Content Administrators from most of the permission roles lists in version 2.3.7.5 for this reason to avoid confusion because Administrators and Content Administrators are special roles thet are not bound by object level allowed roles, so they don't need to be checked to be allowed.
Just as Administrators and Content Administrators do not need explicit permission to upload because they are special roles, Site Editors roles (I think) do not need to be set as a role allowed to upload, they willl still be able to upload even without granting them that permission. So in theory if you have a different role for each site that is used for the Site Editors, if you don't explicitely grant those roles upload permission, then they should still be able to upload within their respective sites. But as soon as you explicitely grant them upload permissions then that would apply across sites even when the role is not a site editor for the other sites.
I do think this was a change in verison 2.3.7.5 where permission allowed roles are now propagated to all sites from the master site and now as type this I am questioning whether that should have been changed. I will change it back because while you can limit a site editor to uploading to only his own site by not assigning the role upload permissions he just intrinsically can upload to his own site anyway, but for other roles I can see where you would want to control their upload permissions on a site by site basis even though other security settings are common to all sites in related sites mode. I changed this thinking I was fixing a bug because other security settings do and should get propagated to the other sites in related sites mode, but now that I think of it these other permissions should be site specific.
Best,
Joe