Blinking Bright Light Effect

Has anyone else had this issue?

In Standard SSF on a late-model Mac, I'm getting a flashing effect on certain mobs. It looks like an overlay box that isn't properly transparent. This box has in the center an opaque bright light which obscures the mob. Ruins the gameplay.

Specifically this effect is on on Spectral Soldiers, Undying Workers, and Statues in the Orchard map and the Labyrinth as well as other mobs.

The thing is,

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THIS DOESN'T HAPPEN IN ARCHNEMESIS.
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In Archnemesis Lab I see small blue glowing orbs on a statue.

In Standard SSF Lab I see garish flashing semi-trasparent boxes with colored bright opaque fuzzy blobs of light in the middle, several per mob.

As I said before, this obscures the action and ruins the gameplay.

I thought maybe it was GPU, RAM or graphic settings, but it seems to be league specific.

Can anyone throw any light on this?

Please let me know,
Thanks.

Did I mention the flashing happens after the mob begins to take damage.
Maybe it's related to bleed damage...