Thank you, Charles, but it's not pasted text. I'm not sure what you mean by "Last line does not have a paragraph mark but a line break." It's a plain and simple complete paragraph, at the end of which I inserted a column break.

And thank you, too, Paul. I am quite certain that the column break is properly inserted at the end of the complete paragraph. I've repeated the entire process several times with the same result. So then I tried text wrapping and the result was even uglier--it forced the last line of the paragraph over to the top of the RH column.
I don't know if this bears on the situation, but the end of the paragraph where I want to insert the column break ends with a footnote. I tried deleting the footnote, but the end result was exactly the same. And I get the same results I'm reporting on two machines, one with Windows 7 and the other with Windows 365.