Extending Tuning page doesn't update with latest parameter value

Coming out of discussion around these issues:

- https://github.com/ArduPilot/MissionPlanner/issues/2354
- https://github.com/ArduPilot/ardupilot/issues/13795

It seems that the Extended Tuning screen (for Copter) does not update the fields shown to the user if the vehicle proactively sends an updated value for the parameter.

Start by reproducing the issue in Mission Planner's Extended Tuning screen for Copter while the vehicle proactively sends a changed parameter value. Trace how the screen receives parameter updates and compare the displayed fields with the latest values. Done means the visible fields update when the vehicle sends a newer parameter value; no files or tests are named in the issue.

Written by the indexing model from the issue text.