Threshold for Photocopy/Pop Art Effect Won't Apply

Hello,

I frequently use Threshold to create photocopy/pop art type effects. Occasionally I will run into images or situations where I can't get the effect to save/apply when saving file, or "printing" file to a new layer via Shift+Option+Command+E. I am an intermediate user, so I may be missing something obvious. 

Here's the look I want to achieve, before applying it to a new layer:

Then, after I hit Shift+Option+Command, here's what I get on the new layer:

There are more mid-tones and black showing.
I've tried placing a Solid White Background as the first layer instead, but result is the same.

Thanks for the help!

Photoshop 24.01.1 on M1 Mac OSX