Hi!

I have updated to:
GNU Emacs 27.2 (build 1, i686-w64-mingw32)
Org mode version 9.4.5 (9.4.5-29-gc43041-elpa

Same issue:
* To the left of :: it's bold

Note: I coudn't find Org mode version 9.5, just 9.4.5

Best regards,
Ypo

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