- Root cause: two prod sites share one Memcached, no WP_CACHE_KEY_SALT
→ cache keys collide → data mixes between sites
- Primary fix: WP_CACHE_KEY_SALT defined via DB_NAME in wp-config.php
(manual on each prod, file is in .gitignore)
- MU-plugin restored: auto-detect backend, versioned keys, auto-invalidation
- Added re-salt fallback via salt_keys(DB_NAME) on muplugins_loaded
- Removed duplicate admin_notice hooks