Apparently this is a known, long-existing and often-reported bug with the Edit function of Apple Contacts, with regards to the "Notes" field. I have experienced this problem for well over the past year, even though I regularly update my Mac and iPhone.
In Apple Contacts if, as I create or edit a contact, and I am typing or editing in the "Notes" field, only the first few lines of existing or new text are displayed, even if I continue to type in that "Notes" field; but if I exit the "Edit" function of that Contact (by clicking "Done"), then I can see the full text of the Note and I continue to see any new text that I am adding to the Note.