7.5.5.2 on MacOS.

The only thing I can think of is macOS clipboard manager somehow interfering with the copied data. Was testing on Linux kf5 UI.

Also, I can preserve the empty row, if I paste with Ctrl+Shift+Alt+V (Edit - Paste Special - Paste Unformatted Text). This opens the Text Import dialog.

You've solved it!

I turned off the clipboard history app I use and now LibreOffice pastes like Excel.

The history app I use is 10 years old and I see that the company that made it apparently is no longer around (or at least their domain no longer works) so I guess I should get a newer clipboard history app.

Great to hear :)

Re: a new clipboard app, I often see people recommending CopyQ, which is cross-platform: https://github.com/hluk/CopyQ