Looking at other posts about the WP customizer (customize.php) not working, I would have thought there was a fix out already. But alas, the customizer does not work for me with mapped domains.
I have an SSL backend and wildcard Cert, so I will not be setting admin or login mapping to anything other then original domain. Frontend is forced http as I have varnish serving http frontend.
Crossdomain autologin doesn't make any difference. I'm using 'Make' as the theme in use and would like to keep using it. So It is left off.
Currently, my site is open to support access for another issue if you'd like to check out what I mean.