I have narrowed down the issue ...
It's related to stacks with a period / "." in the name
Original name 1. Projects - blank
Renamed to Projects - shows
Renamed to A. Projects - blank (thought it was the number)
Renamed to A Project - shows (realised it was the special character)
Renamed to 1 Projects - shows (numbers are OK)
Renamed to 1-Project - shows (hyphen OK)
Have replaced "." with "-" for all stacks - all displayed
FYI notebooks with a "." that aren't a stack top level folder display correctly
