I was not aware that problems like the described even existed. I remember seeing a dialog once when browsing BasicUI, asking which of the two available sitemaps (on my system at that time) I wanted to visit. Did the problem get solved after you set the default sitemap or after you synchronized the filename and the sitemap name as @Benjy suggested?
After they pointed out the problem I chose to rename my sitemap
from:
default.sitemap
to:
_default.sitemap
That way hopefully the default functionality of OH doesn’t change again and I will never burn time on this again.
I could have reconfigured PaperUI>Configuration>Services>UI> Basic UI and Classic UI to both point to the sitemap name of my choosing but figured if/when I migrate to a new install I would forget and burn hours again.
Trying to keep things as “Out of the Box” compatible as possible (but this time the box changed ).