Go to a mirrored list like https://lists.sr.ht/~sircmpwn/alpine-devel and click on an archive. Hover over the "Reply to Thread" button at the bottom of the thread and the mailto link has "mailto:mailto:___".
I sent in a patch for this: https://lists.sr.ht/~sircmpwn/sr.ht-dev/patches/10396. I'm relatively sure this should fix the problem. Still unsure of why mirrors specifically are having the problem.