Why does the separator "::" work with headlines?
It doesn't seem to have any effect for me, it gets handled as part of the headline, both in fontification and in exports:
Which is, as I understand it, as it should be, since it doesn't have any special meaning in headlines.
In your case it seems indeed to be getting fontified as if it were a list. What version of Org do you have? I am using 9.5 on Emacs 27.2.
--Diego