Select and Mask Views Don't Show Hidden Areas - Photoshop 20.0.5 macOS Mojave

I do composites. I use S&M to cut away parts of the top layer to reveal the underlying image. Both layers are currently in Normal blend mode. I open S&M and use Overlay view and the Quick Selection tool, then output to a Layer Mask to evaluate my progress. When I reenter S&M to continue the selection process, none of  S&M's views let the underlying layer show through (so I can see what to select next), even though the Overlay layer opacity is ~50%.

Just noticed! Output Settings (just above Decontaminate Colors checkbox, near bottom of S&M panel) is MISSING.

If I paint/select in any area of the red overlay, the hidden parts of the image show up, but I can't select what I can't see. See my screenshot: I drew a big blobby area on the left side to show how the masked parts of this layer are still there, just not showing through in any of the S&M View options (Overlay, Onion Skin, Layers). Also included: a screenshot of the image when NOT in S&M. It looks normal to me.

Here's what I've done so far:

• Reset the S&M panel
• Reset all tools
• Restarted PS and LR
• Done a lot of online research

Please help if you can.

Mac OS Mojave (latest) and PS/LR CC 2019 (also up to date).

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