My plaudits to Jasell____ who seems to have stumbled on the solution to my problem. As described, I went to Settings ->Apps -> Default Apps -> Screen Snipping.
I had looked here previously and after scrolling a long way down to find "MS_SCREENCLIP", made sure that "Snipping Tool" was set as the default app which makes logical sense. But as Jasell____ suggested I switched the default app to "Screen Snipping" not really seeing the difference between these two options.
Then, on right-clicking the taskbar Snipping Tool shortcut and clicking "Take a new screenshot" and drawing the box, volià, the Snipping Tool editing/save window popped up! This is excellent news. This is the behavior I've been used to for some years in both Windows 10 and 11.
There is still the annoyance of Win Key + Shift + S not behaving the same way - it just causes the notification to pop-up which must then be clicked to open the editing window. But this has been the case in both Windows 10 and 11 for a long time but since I don't usually take a screenshot that way I haven't let it bother me too much.
And there's still the bug of the window not being positioned where I placed it previously - the Snipping Tool seems to have a mind of its own as to where to position itself on the display sometimes even very close to the top making it difficult to drag it down. This is surely a simple thing the MS could fix unless it's something peculiar about my graphics settings.
Still, I'm most grateful for Jasell____'s idea.
Shaun.