The Instant Retry won't retry transactions that were failed due to network issues, timeouts, and so on (error codes 500, 503, 504, etc.), that is because we cannot conclude if the requests received on the provider's side or not.
If the feature will retry the payments with another provider, we could risk double charges, as in some cases, it can be possible that the first provider did receive the requests without responding back due to downtime/network.