This maybe not entirely related but I noticed that enemy unit frames (not nameplates) don’t show the cast bar anymore. I don’t recall changing anything on my end, additionally some of the people in my guild that are little less high speed brought this up during launch weeks. But were told to rely upon nameplates.
Maybe, these people who aren’t interrupting have been relying upon those now-missing unitframe cast bars and then play without nameplates? Sounds like a real stretch/excuse but seeing how people managed to slip through the cracks in legion when acquiring artifacts (like they reach level 110 without a weapon at legion launch), nothing surprises me anymore.
Pretty sure this is what I was talking about: Enemy cast bars
And looking down that more, looks like it could be a mostly one and half source of an issue, caused primarily by addon “knowing best” and maybe a cvar just getting changed in pre-patch.