RSSI freezing at last value on OSD when it should go to zero, can't see RxRSSI on Mission Planner

I am using Dragonlink for C2 and Telemetry. I am using PPM from the Dragonlink Rx to the Pixhawk; The Dragonlink Rx is set to no pulses as failsafe setting. I have set up RSSI as a PWM channel (7) and this is working properly. The RSSI values in the dataflash logs look fine, including when the Tx is switched off it goes to zero.

Unfortunately for some reason Mission Planner, whether over dragonlink or plugged in via USB, does not report ch7in or RxRSSI, they just show zero at all times (same with all other chXin too). I have tried changing SR1_RC_CHAN and it goes back to zero after reboot. I think Mission Planner might be setting it to zero?

Anyway, not being able to see RxRSSI in Mission Planner, I’ve set it up on my OSD (Minim OSD 2.4) instead. This works fine, except when the RxRSSI drops to zero. For example if I power up the aircraft with the Dragonlink Tx turned off, RSSI correctly shows 0% in the OSD. When I switch Dragonlink Tx on, it goes to 100%. When moving to further distance, the RSSI varies in a way that matches the dataflash log. However, if I switch off the Tx, or get a lock out in range test mode, the RSSI on the OSD sticks at the last known value e.g. 100%.

Any opinions on whether this this an OSD bug or an ardupilot bug?

This situation is far from ideal as I cannot see zero percent when C2 has been lost!