Okay, I have an odd one.
The search results page is not showing promoted result blocks.
If the query is tested from the Results Web Part, the test results display showing the result blocks.
Platform: SharePoint 2019 (Enterprise Server) with Search Service Application with two result blocks
I feel like I'm doing some 1+1=3 somewhere in my configuration and checklists, but I have not tracked it down yet. (After decades of deep work with SharePoint Server - this is embarrassing.)
Any thoughts about the glitch (and how to troubleshoot it)?