The system function works as defined, which is for the current database user

When connection with a domain account with sysadmin role:

select is_member('UserRole'), user_name(), suser_sname()

execute as user = 'Carl.Federl'

select is_member('UserRole'), user_name(), suser_sname()

revert

go

Output is:

0 dbo AAUSA\carl.federl

1 Carl.Federl Carl.Federl