@rad - If you don’t return anything in a callback, you are technically returning None which will get transformed into null and Dash should update the output property as null, which is valid for some cases (e.g. None as a dcc.Dropdown.value is OK - it means nothing is selected and None as a children property is OK too, it means no element) but not valid for other properties (e.g. options of a Dropdown can’t be None, as you saw here: Debug something not updating on particular callback, inscrutable JS "promise" error - #4 by rad).
In some cases, it will clear the output, rather than leave the output to be unchanged.