snap-store-proxy: can't add override for snap in brand store

I cannot add an snap/rev override for snaps in the brand store used by my devices.

For example, the ce-team-test store has test-revision rev 8 on stable (armhf). And I can install it on the device through the snap-store-proxy (SSP). But I cannot add an override for this snap.

On the SSP host
---------------

root@snap-store-proxy:~# sudo snap-proxy override test-revision stable=8
Failed to set override:
No snap named 'test-revision' found in series '16'.

root@snap-store-proxy:~# sudo snap-proxy check-connections
http: https://dashboard.snapcraft.io: OK
http: https://login.ubuntu.com: OK
http: https://api.snapcraft.io: OK
postgres: localhost: OK
All connections appear to be accessible

On the device
-------------

knitzsche@ubuntu:/snap/pi-kernel/current/meta$ snap info test-revision
name: test-revision
summary: Boilerplate2
publisher: Kyle Nitzsche (knitzsche)
store-url: https://snapcraft.io/test-revision
license: unset
description: |
  This is my-snap's description. You have a paragraph or two to tell the
  most important story about your snap. Keep it under 100 words though,
  we live in tweetspace and your description wants to look good in the snap
  store.
snap-id: Hd1B6bexX487FYwOzk8ChZyja7A10QxS
tracking: latest/stable
refresh-date: today at 16:31 UTC
channels:
  latest/stable: 0.3 2021-03-09 (8) 8kB -
  latest/candidate: ↑
  latest/beta: ↑
  latest/edge: 0.1 2021-02-08 (1) 8kB -
installed: 0.3 (8) 8kB -

Same thing for another snap in the ce-team-test store
-----------------------------------------------------

root@snap-store-proxy:~# sudo snap-proxy override demo-game-0 stable=1
Failed to set override:
No snap named 'demo-game-0' found in series '16'.

But, I can add overrides for snaps in the global store
------------------------------------------------------

root@snap-store-proxy:~# snap info learnit
name: learnit
summary: Learn through flash card tests
publisher: Kyle Nitzsche (knitzsche)
store-url: https://snapcraft.io/learnit
contact: <email address hidden>
license: LGPL-3.0+
description: |
  Learn through flashcards from external card files
snap-id: OHILDGdP7v0yZmkQnvGyXnGD5t2mZZ1g
channels:
  latest/stable: 1 2021-07-22 (23) 1MB -
  latest/candidate: 0.2 2017-05-17 (2) 696kB -
  latest/beta: 0.2 2017-05-17 (2) 696kB -
  latest/edge: 1 2021-07-23 (25) 1MB -

root@snap-store-proxy:~# sudo snap-proxy override learnit stable=23
learnit stable amd64 23 (upstream 23)