SOLVED, TL;DR: Rocket.net's infrastructure strips tilde characters (~) from query parameters at the server level before they reach WordPress/PHP. This broke WooRewards (AKA MyRewards - the free version of the plugin) referral links. After months of debugging and working with their engineering team, they confirmed this is a platform limitation they cannot change. Solution: Switched hosts to Cloudways - referral links now work immediately.

Disabled all caching (WordPress, Cloudflare, server-level) Modified .htaccess with cache bypass rules Created MU-plugins for early session initialization Tested with multiple referral plugins (same issue across all) Cookie whitelisting (only partially helped)

Definitive proof it was server-level: Created debug logging that showed when visiting ?~=abc123&foo=bar, the URL arrived at the server but $_SERVER['QUERY_STRING'] was empty in PHP - the tilde parameter was stripped before reaching the application layer. Lesson learned: If your referral/affiliate plugins use non-standard query parameter formats (especially tilde), verify your host doesn't filter them. Standard parameters like ?ref=CODE worked fine, but WooRewards' referral link format was incompatible with Rocket.net's infrastructure.

Working solution: Migrated to Cloudways] - both WooRewards tilde links and standard referral code plugins now function correctly on first visit.