With earlier versions of Mint Cinnamon I would see one icon in the panel for each file or doc I had open.
With Linux Mint 20 Cinnamon (and with the earlier 19.3 Cinnamon) they are nested--for example, whether I have one document open or several, the Libre Office icon is highlighted; if I have a file open, a file icon opens in the panel, but only once, and I will see the individual items only when I scroll over the icon.

I dislike this, partly because I am a creature of habit, but also because I find it less efficient. Is there a way of returning to the previous behaviour?

I am currently running Mint 20 Cinnamon 64 bit on an HP netbook.
Thanks!

Have you examined the panel properties and settings?

I saw nothing in the panel properties that made a difference, although I fooled around with the applet "Grouped window list" and confused my settings temporarily!