Currently when XC returns an HTTP 421 error due to HTTP/2 connection re-uses:

Connection Coalescing: In HTTP/2 or HTTP/3, the browser or client reuses an existing TLS connection for a different domain, but the target server is not configured to handle that second domain on the same connection.

This applies to HTTP LBs where one has a wildcard certificate and another has an explicit certificate. The client might send the request for the explicit to the wildcard as it re-uses the connection. XC HTTP LB responds then with an HTTP 421 with the default error page even when custom error pages are configured.