Hi @TomBole . I just got some time to jump back in. Not sure how ESRI defines a bug, but this may qualify. I have a dumb hack to darken background imagery wherein I place a polygon covering my entire map area and set it to a light grey fill. Then Blend --> Multiply. (NOTE! This was the problem - Multiply, not the layer in Blend Mode = Normal)
This was specifically to enhance the World Hillshade and Terrain Hillshade Tile Vector Layers from ArcGIS Living Atlas. I checked that layer prior to posting, but apparently not thoroughly - it is the culprit. But, it seems unintended behavior. The clipped map displays in the Map but NOT the Layout.
So I am unable to retain that layer. Once removed, the Layout is fixed.
Thanks for following up. Happy to submit a report to ESRI if you think it's warranted.