Since admin requests aren't routed through the cache frontend, the global variable isn't available. Use defines for these overrides instead.

Unfortunately does mean that settings are split, but alternatives weren't appealing:
* store settings into an option via the front-end proxy (front-end writes are bad)
* store in Redis (could lose the option, randomly revealing a UI)
* pass all wp-admin requests through the proxy with early bailing to avoid breakage seems overly complicated and fragile/prone to bugs.