I have narrowed it down to the "save form entries" option hidden in the advanced settings of Edge.
If this is disabled it works without issue. Further to this if I clear the "form data" it also goes back to working.
Note that originally over time I built up a collection of saved values and have found not all forms cause the issue.
If you go to the following URL
and enter test, test, test and click login, which will of course fail.
Now test should appear on
as explained when you open the dropdown list and click in the filter box.