Copying in Excel not appearing in clipboard manager

macOS 10.14.6; BTT 3.202 (1378); Excel 16.29.1.

I'm not sure when this started – it used to work fine – but I often find that text I copy in Excel does not appear correctly in clipboard manager: I get a row with the time & Excel icon but just "-" as the label and there is no preview when I click on the row. I can paste these entries, but without a preview I don't know what they are! I haven't figured out the pattern, but it might be when I've copied from within the formula bar…