You should be able to just omit the third parameter, and unmatched values will remain unchanged.
However, the way you've coded it by specifying circuit_group as the third parm should work as well. So I suspect there is something else going on. Can you try omitting the third parm, and if that doesn't work, can you post a sample demonstrating the problem?
Seems to be working for me:
LOAD * Inline [
Upper(ApplyMap('circuit_group_map', circuit_group, circuit_group)) AS [Circuit Group]
Community_192828.qvw 144.8 K