Thanks for your suggestions.
I tried switching fonts, but with no joy, and after I saw in Font Book that I had two copies of Verdana I resolved them, also with no joy.
And I spoke to the local witch, but she doesn't have her hexes edited by anybody, so I can't try that idea.
Oh, well, it only happens in QuickLook, not the spreadsheets themselves, and although it's a nuisance, it doesn't cost me anything more than a bit of annoyance, so I'm not interested in getting any deeper into the issue than I've already gotten.
Update: I decided to tread on new ground and see what I could see with a hex editor, and I'm afraid that I couldn't see anything that meant anything to me.
None of the cells with the artifact appeared on the text side of the editor. Does that mean anything to you?