Hi Borate,
Thank you for your reply. Yes I'm using v8.63.
I've done some experimentation, and have found that as per your advice, when keeping the same resolution as the source file the text is displayed correctly.
However, when I reduce the export resolution to very low (which I have to do sometimes to send the video to others), it's incorrect when Letterbox is selected, but is ok when Crop and Zoom is selected (which isn't always optional).
So it's a bit annoying that what you see in the Sequence Preview is not always what you get. But at least I know how to fix it now.
Thank you again!