I’m having the same issue.
Video clip on YouTube: https://youtu.be/iuViTzJrLqA
AMD Radeon 5700 XT w/8GB ram… in a Razor Core X eGPU housing w/Thunderbolt 3 cable. Brand new install of Diablo 3 on MacOS 11.2.1 (Big Sur). MacBook Air (Late 2018) dual-core i5 w/16GB RAM, fully up to date w/official Apple graphics drivers. Plays other Blizzard games w/out any graphical glitches.
Some shader bug when calculating effects on surface textures over adjacent geometry. It’s almost like the surface normal of the geometry for baked-in lighting has overflowed and been reversed, because the pixellated chunks that shimmer are subtracting the effects of illumination entirely, even from the simpler “halo blob” drawn behind the lit object. The object itself does not have any lighting applied to it, it is plain and lit at the default “global illumination” level. The pixellated shimmering effect is only visible at the edges where the light-emitting object overlaps the “halo blob” backlight.
I’m guessing those backlights are in game as simple 2D geometry with a starburst (or smear, for cave entrances) “texture” on it, with an alpha that quickly ramps up to complete transparency, but is painted into a buffer layer reserved for sources of light – so when the projected light from the object interacts with it attempting to additively increase the illumination, it actually ends up cancelling or subtracting the light from it, thinking “oh, that geometry is facing the other way, do not apply lighting to it.” This is why the shimmer doesn’t happen to everything, only to object’s “projected light” shader checking its boundaries in screen space and accidentally destructively interacting with other “light”.
Lights in D3 tend to have a mix of contributing illumination to dark surfaces, but also “painting over things” with a see-through color contribution, so the light itself has a kind of weightiness or volume.