unable to create new sheet in tab bar

mac mini late 2012 v 10.16.7 Catalina

Numbers v11.1 (Note: my mac mini can't update to v11.2 because I can't update beyond Catalina.

Problem:

When I click on the "+" to add a new sheet (sheet 2) in Numbers, a new sheet is displayed but there's no new tab for that new sheet. Only the sheet 1 tab is displayed, there's no way to navigate to Sheet 2.

What I've done:

Under view I have 'show all tabs' selected.

In addition to clicking the "+" to add a sheet, I also tried to 'insert sheet'. Still no tab.

Temporary Solution:

The only thing I can do is go to the top edit bar > Window > select "Show Next Sheet" then I can see Sheet 2, with only the 'Sheet 1' tab showing in the tab bar. If I click on Sheet 1 I go to that sheet but there's no 'sheet 2' tab, I have to go back to the Window > Show Next Sheet.

Question:

Was this some kind of problem with 11.1? If not, any suggestions.

Quite baffling.