https://docs.python.org/3/library/enum.html#allowed-members-and-attributes-of-enumerations

> The rules for what is allowed are as follows: names that start and end with a single underscore are reserved by enum and cannot be used; all other attributes defined within an enumeration will become members of this enumeration, with the exception of special methods (__str__(), __add__(), etc.), descriptors (methods are also descriptors), and variable names listed in _ignore_.

Looking at the code _member_names has the list of member names internally that stores the members. I guess with a callable assigned to TEST it becomes a method and is not added as a member as per the above doc to skip descriptor as method.