Setting SR0_EXTRA1 on APM3.3 Plane

I am struggling with setting this parameter on APM3.3 Plane from current version of Mission Planner. Here is what I find:
a) Connected by USB to PC
b) Planner > Telemetry Rates > Attitude dropdown changes on screen (from 6 to 10) but doesn't change on APM
c) Full Parameter Tree change of SR0_EXTRA1 makes change (from 6 to 10); Change holds on refresh - but does not reflect back up in the Planner > Telemetry Rates view
d) When disconnect and reconnect SRO_EXTRA1 reverts to 6 in Full Parameter Tree and Telemetry Rates, but SRO_EXTRA2 in Full Parameter Tree changes to 10.
e) Same behaviour with APM Reset (on USB Connect) ticked ot not.
I am not overly familiar with this architecture yet, and it could well be user error, but this behaviour feels repeatably inconsistent and I am wondering whether there is some cross over of variables in the code.

[Edit] Note as per edit in first line - it is APM3.3 Plane

Reproduce the APM3.3 Plane behavior described in Mission Planner's Telemetry Rates view and Full Parameter Tree, focusing on SR0_EXTRA1 and SR0_EXTRA2. Compare values after editing, refreshing, disconnecting, reconnecting, and resetting; done means the parameter remains consistent between both views and across reconnection.

Written by the indexing model from the issue text.