Responding with an already passed date in the expired header (earlier than the Date header value) makes no sense and would be a sign of some sort of misconfiguration.
My client would treat such response as “already expired” and not cache it, thus spamming your ESI service with requests.
Of course, I could always hard-code a proper cache time, but as you rely on caching much instead of rate-limiting (which i really find great you do), you need 3rd party devs to use the caching mechanisms, but they won’t if they dont work for them.
It happens most frequently with Universe routes - Systems, Stargates and so on
See that Screenshot, for example, was made with that request