After seeing the posts about the API changes supposedly fixing the missing user and guest sync problems, I tried my scales. Issues weren't resolved.
Reset the scale and re-added myself as guest (wife is main owner). For a couple of days it worked, despite displaying the small 'x' during my upload.
However, this morning it didn't recognise me. I tapped through a couple of '?' until I found myself but it failed to upload to Garmin Connect.
Sigh