> Use of the "h" register slices (bits 8..15) by compilers is thankfully pretty rare -- otherwise this would have been noticed much sooner!

It's actually pretty easy to get compilers to use those, you mainly need a bunch of narrow accesses to neighboring memory. The oodle post contains a godbolt link to pretty ordinary c code triggering this.

I'd guess that you also need some other conditions (multiple in flight stores, high boost speeds) to trigger this.