Upfront not working on a mapped domain if accessed with super admin

On a multisite install, if I access to this subsite with a mapped domain http://blackflag.imsitepros.com from the admin dashboard logged in as super admin, try to access Upfront then I'm redirected to the mapped domain http://blackflagtrading.com/?editmode=true but it looks like there is no session active so is not possible to edit with Upfront.

So I went ahead and created a second admin for that subsite, logged in using the mapped domain and then enabled Upfront and I was able to use it.

The problem is just when someone tries to make changes to upfront from the subdomain to the mapped domain.

Thanks in advance!