The only instance I've ever seen this personally involved a case where a gedcom has a note in common for all people in the gedcom.This is allowed in the gedcom standard.When this was imported into a data file then a change in that note field for any individual showed up in the note field for all individuals.
I was able to edit the gedcom file to remove that common note and re-testing showed that this fixed the behavior after import.
Other testing showed that creating a new note field and copy/pasting from the problem field to the new field also resolved the problem IN THAT PARTICULAR CASE. Then the problem field was deleted. [Caution, always keep a good backup before experimenting with data!!!!]