I created a custom theme (only login page) which has as a parent the base theme.
When I select this custom theme in the master realm, the theme is applied on the next login. Everything fine so far.
Now I select the same theme in my first non-master realm. Exactly the same theme configuration as for master.
When I now go to the login page for the realm, I still see the base theme. It just seems to load slower (first mainly white background and odd positioning of the form) after a bit, the default theme properly loads and shows.
In the network logs in the browser, I don’t see anything indicating a failed load of resources, or a second attempt to a fallback base theme.
Any ideas what could be wrong?