Oops, spoke too soon. A closer look in QBooks showed that the "/n" is showing up as text between lines.
So, I tried adding a " " (1 space), and that made the "/n" disappear, but lines continued as a single long line with no line breaks.
Next I tried adding " " (5 spaces), and that improved it, but not on all lines.
So finally this worked for me:
vCM = @Replace(CustomerMessage, @NewLine(), @Text(20, " "))
This provides enough space chars to force the text to the next line. I think this will work for us.
(Another option now being considered by customer is to make the CustomerMessage box wider (aargh!) in which case I will just need to increase from 20 to another larger number).