As of 2/13/20, I’ve noticed a number of missing spell effects. For instance, the breath on Vexiona (run in LFR if that makes a difference) when she is in her sky phase, or the silencing effect on the Grand Empress world boss in Vale of Eternal Blossoms, do not appear. Likewise, when I look at mage portals (or those found in Boralus or the Stormwind portal room,) the portals themselves appear, but the images of the destinations within them are missing.
I’m running Mac OS 10.14.6 on a 2014 Mac Mini, with an Intel Iris 1536 MB graphics card. These issues were not present in the first weeks of 8.3, but seem to have come with some later mini-patch.