I've done something... stupid to fix it for now. Since I suspect their JavaScript is screwing it up, but I can't find what would be preventing it... I've thrown scripting at it to detect a hash, see if the matching ID exists, and scroll to it.
Since scripting disabled the new posts work... Another stopgap until I can get knee-deep in the total skin rewrite.
STILL not working... no, it couldn't be... does display:flex somehow magically disable hash links in this usage case? The numbers returned by my top calculation, rect + scroll, etc, are total BS. Or is flex not returning its height/size properly?