After some testing along with others, it appears as though certain weapons (such as Nightfall) are incapable of triggering during the global cooldown.
To test this idea, I logged a bunch of hits on a blasted lands mob, and logged it here:https://classic.warcraftlogs.com/reports/rFwMG7fVTv2NcAxy/#fight=last
In this fight, I used Sense undead or seal of the crusader right before the end of each swing timer. Across 207 hits I got 1 proc off of nightfall (on that swing I was distracted and was late on putting on the GCD).
I never got a proc of nightfall when my character was waiting for the global cooldown to finish.
According to Nightfall’s proc rate, this result is ABSURDLY uncommon, and this GCD interaction is likely to blame for the reported low proc rate on Nightfall that blizzard debunked a few months back.
The proc rate is fine, it just can’t trigger if you are on GCD.
This bug may apply to other proc effects, as I assume that Nightfall is not the only weapon coded this way.