Trying to get user info using iControl REST on 11.6 for a local user created in a partition other than Common

We created a partition called "Users" and create all local users there. I want to see if a user name already exists using REST. It seems I can only get info on accounts created inthe default /Common partition. I have tried adding the partition to the uri and using the filter option with no luck.

Here is what I get using curl:

curl -sk -u admin:xxxx 'https://bigip.edu/mgmt/tm/auth/user/wgeiger1?ver=11.6.0&$filter=partition%20eq%20Users'

RESULTS: {"code":404,"message":"01020036:3: The requested user (wgeiger1) was not found.","errorStack":[]}

curl -sk -u admin:xxxx 'https://bigip.edu/mgmt/tm/auth/user/wgeiger1?ver=11.6.0'

RESULTS: {"code":404,"message":"01020036:3: The requested user (wgeiger1) was not found.","errorStack":[]}

curl -sk -u admin:xxxx 'https://bigip.edu/mgmt/tm/auth/user/wgeiger1'

RESULTS: {"code":404,"message":"01020036:3: The requested user (wgeiger1) was not found.","errorStack":[]}

curl -sk -u admin:xxxx 'https://bigip.edu/mgmt/tm/auth/user/~Users~wgeiger1'

RESULTS: {"code":404,"message":"01020036:3: The requested user (/Users/wgeiger1) was not found.","errorStack":[]}

And for a user in the /Common partition:

curl -sk -u admin:xxxx 'https://bigip.edu/mgmt/tm/auth/user/testUser'

RESULTS: {"kind":"tm:auth:user:userstate","name":"testUser","fullPath":"testUser","generation":0,"selfLink":"https://localhost/mgmt/tm/auth/user/testUser?ver=11.6.0","description":"testUser","encryptedPassword":"!!","partitionAccess":[{"name":"EMMS","role":"manager"},{"name":"RAD","role":"irule-manager"}]}

Is this a bug? More likely I am doing this wrong - any ideas?