Snap store proxy 502 errors from api.snapcraft.io

[snap-proxy-user]

We have:
- Snap store proxy deployed on a single machine
- ~70 machines connecting to it, but never at the same time
- Snap proxy status shows all green, all services are OK

Snap commands will fail intermittently on clients ("snap store unreacheable"). The only useful log from the proxy is this one from snap.snap-store-proxy.snapdevicegw.service:

 Max retries exceeded with url: /api/v1/snaps/auth/sessions (Caused by ResponseError('too many 502 error responses'))

And this one from snap.snap-store-proxy.nginx.service:
 "POST /v2/snaps/refresh HTTP/1.1" 499 0 "-" "snapd/2.67-0.el8 (series 16; classic; devmode) rhel/8.10 (amd64) linux/xxx.x8

Some clients get this error sometimes, others more rarely.

How can we approach debugging this further?

[snap-proxy-user (mwam-ubuntu-sso-test)]

Is this forum followed at all?