Edits to chord symbols not linked

Likely related to #279175: Chord symbols entered into score not parsed in parts and probably #278972: Chord Symbols not mapping from score to parts consistently (since I was able to reproduce that while following essentially these same steps, although I don't at the moment remember how).

1) default empty score
2) enter four quarter notes into first measure (so mmrests aren't a distraction)
3) enter chord symbol C on beat one
4) generate parts
5) double-click chord symbol, add a "7"
6) view part

Result: still says C. Further edits in either the score or part are not linked.