Class Member List does not show friend operator parameter list. doxygen-1.9.1 does not generate the warning but doxygen-1.9.2 and later does, including trunk 1.11.0 (4c99c07*).
/**
* Test docs
*/
class Test
{
/**
* operator== docs
*/
friend bool operator==(Test&, Test&);
};
Doxygen-1.9.1:

Doxygen-1.9.2 and trunk (4c99c07*):

Testcase:
testcase.zip