I thought it was a bug, i experienced it today as i was raiding ICC25.
I made a bug tracker that you can find here :
https://www.warmane.com/bugtracker/report/112354
If you go visit that URL, you'll find down in the comments the answer directly from the devs :
"Intended change, new procrates are taken directly from LK classic. Certain AOE spells have reduced chance to proc Clearcasting. It will show up in the nexth changelog."
It's the end of an era... and an effective nerf to our overall dmg by 1k to 2k depending on the fight.
RIP.