They’ve confirmed that certain effects actually cannot proc while under the effects of GCD. But, they’ve investigated further and confirmed this,
“We also discovered that there were specific provisions built into the original game to force this to be true. However, we discovered that this is true only if the spell or ability that triggered the GCD was flagged as a magic-based attack (or more specifically, spells flagged in our data with a defense category of “magical”).”
This also affects physical abilities and spells as well, with those abilities and spells incurring the GCD not blocking procs from going off during GCD,
“An example of this is a Warrior’s Hamstring ability. This has a defense category of “physical”, thus you can still see Nightfall procs occur while you are on GCD following the usage of Hamstring. Alternatively, Demoralizing Shout has a defense category of “magical” and you will not see Nightfall or certain other items proc during the GCD immediately following that spell’s usage.”
The team confirms this was a quick of the original game, and something they do not consider a bug.