Thank you - indeed simple and a fast response.
I was just about to add that I had eliminated bindings from the equation via removing all of them on the listbox (bindtags([])) and using selectionSet() to add one before changing the text in Entry with same outcome when I saw your reply. I have tested with -exportselection 0 & 1 on the Entry Widget but 0 on the Listbox and it indeed behaves as expected(by me).

If this is behavior is documented somewhere I clearly missed it (along with -exportselection being on by default which is documented) - I assure you I did multiple searches and lots of 'what if I try' before posting.

Again thank you for your expertise and time to share it!