Cannot disable "Mission" flight mode during configuration


I am configuring the system in QGC to switch flight modes using different thresholds on channel 5.
I can disable “Acro” mode (choose Disable in drop down menu), but not “Mission” mode. Drop down menu for Mission is grayed out (not accessible). Is it normal ?

I tried moving the threshold for Mission all the way to the right, but it scews up all modes below Mission in the list.
i.e. if I set Mission mode threshold higher than Hold threshold, then Hold never gets activated even if channel 5 signal is higher than Hold mode threshold.

I am not sure how Mission mode works yet and don’t want to activate it occisdentally as I switch through modes sequentially on my radio. Channel 5 is currently controlled with two switches (3-position + 2-position). I can always reprogram the radio to never output the signal corresponding to “Mission” threshold, but it seems strange to me that I cannot simply disable it in QGC. Maybe there is a reason?

Pixracer FC.
PX4 FMUv4 - downloaded through QGC some time around September 10.
QGC - latest dayly build downloaded Sept 24.
Kill switch and RTL switch are on separate dedicated channels.
Other flight modes are on channel 5.