When I was doing the wiki
buff lines page (which was all about stacking) I mostly relied on live data, because
very few spells ever had how they stack changed.
You can easilly check any spell on live by using Allakhazam's Lucy database, and if you check:
https://lucy.allakhazam.com/stacking...12&source=Live
https://lucy.allakhazam.com/stacking...16&source=Live
... you'll see that (on live) they do stack. Quivering Veil doesn't stack with Harmshield, but that's about it for this era.
So ... either this is one of those very rare spells that
did change, Nilbog learned about it, and this is a special classic fix, or ... this is a bug.