Why Does the Mercury Playback Engine Setting Change Opacity?

Lately, I have been working with a lot of motion graphics with black rectangles set to 82% opacity. 

We discovered that it looks correct when the Premiere Pro Project Settings (AND, when rendering, the Renderer setting in Media Encoder) are set to "Mercury Playback Engine Software Only". When it's set to "Mercury Playback Engine GPU Acceleration (Metal) - Recommended", the look changes: the opacity looks a lot lighter, the black looks grayer. Please see the attached examples. 

Can someone please explain why this happens? Is this normal/expected Premiere/Media Encoder behavior, or a bug?

I'd love to take advantage of [what I assume are] the faster exporting speeds with the GPU Acceleration on, but I can't—every time I have to render a video with one of these graphics in it, I'm forced to go Software Only instead [which, from what I understand, is slower, right?]. 

(Just in case: please note that, in the software only example below, the black box is definitely not completely opaque. It has some transparency.)

SOFTWARE ONLY:

GPU ACCELERATION (METAL): 

What is this madness? 

Thanks in advance.