Thanks for the answers. Now this really is strange.
If I select the store and go to ‘My Library’, the PC spends a few seconds trying to search and finds nothing.
However, if I go to the top and, instead of selecting ‘All types’, select ‘Games’ I get a list which includes MSFS. If I select the ‘Apps’ Option then my MSFS digital ownership is listed.
Previously the ‘All types’ (default) option would have listed these so it appears it’s a bug with the Store app.
I did try resetting and it made no difference.