I have found the solution for me. It’s an Unreal bug, in 4.24.0 and 1 and 2, which I’ve submitted.
My game didn’t start because I had the Editor Scripting Utilities plugin enabled. I looked in the built game’s log and discovered that EditorScriptingUtilities was erroring and shutting down the game at launch. Of course, it makes no sense that the presence of a plugin should do that to a built game that did not error during build. I disabled the plugin, restarted Unreal, rebuilt my game, all good.
So maybe other plugins as well, especially if experimental or just poorly supported by Epic, can cause no-launch scenarios. Look in the log files and they should show what caused the application to exit.