Mirror mavlink stream on serial port Broken

#### Issue details
in mission planner, ctrl+F, mirror mavlink stream to output serial doesn't work anymore (to com1, 57600 or 115200, write access ticked or not).
I use this feature of outputting mavlink stream for antenna tracker and externall small converter use and this feature is so nice (yaapu debug/test/improvement and so on)
If it could be be debugged, it will be super nice.
example use case: https://youtu.be/R3f62-rtvXI

#### Version
1.3.68.1 build 1.3.7160.16169

#### Platform
[ ] All
[ ] AntennaTracker
[ ] Copter
[x ] Plane
[ ] Rover
[ ] Sub

#### Airframe type
plane in sitl

#### Hardware type
sitl

#### Logs
no logs

Reproduce the issue in Mission Planner using Ctrl+F and the “mirror mavlink stream to output serial” option with COM1 at 57600 or 115200, first in Plane SITL. Trace the serial-output path from that setting and verify completion when the MAVLink stream reaches the configured serial port for the antenna tracker or converter use case.

Written by the indexing model from the issue text.