Ubuntu 20.04 installation gave my terminal all squares with 4 characters in them
Solution 1:
I managed to solve it by going into Preferences > Compatibility and selected Ambiguous-width characters: Wide. It seems it was set to Narrow.
Solution 2:
Prior to the upgrade, I'd selected a custom font in Preferences > [Profile] > Text, and ticked the Custom Font checkbox.
I suspect the upgrade retains the state of the checkbox but doesn't retain the font selected.
Simply selecting a custom font or unchecking the Custom Font checkbox worked for me.