Switching into or out of pitmode doesn't work with Eachine Nano v3 VTX (IRC Tramp) + softserial+ INAV 5.1

[cmanley]

Current Behavior

If I disable pitmode by switching to a power level on the VTX using the button, then I am able to switch between power levels 1 (25mw) to 4 (400mw) from INAV. What INAV is unable to do however is switch out of pitmode when arming or into pitmode when disarming. Unfortunately I'm stuck after many hours of trying and unable to find much documentation in the wiki on VTX communication and possible issues and solutions. This may be related to https://github.com/iNavFlight/inav/issues/5844 and/or https://github.com/iNavFlight/inav/issues/5896 and/or https://github.com/iNavFlight/inav/issues/7937

Steps to Reproduce

1. Connect VTX RX to softserial 1 TX.
2. Set vtx_power = 4 in INAV.
3. Set vtx_low_power_disarm to ON in INAV.
4. If the VTX is already in pitmode, then arming won't make it exit pitmode.
5. If the VTX is not in pitmode (after setting a power level using VTX button), then INAV can change the VTX power level to 4 on arm, and back to 1 on disarm, but it won't put the VTX into pitmode.

Expected behavior

INAV can control pitmode state of this VTX that uses the IRC Tramp protocol.

Suggested solution(s)

Perhaps more documentation if some issues/solutions are known.

Additional context

# version
# INAV/MATEKF411TE 5.1.0 Aug 19 2022 / 12:19:47 (76f22b25)
# GCC-10.2.1 20201103 (release)

serial 0 2 115200 115200 0 115200
serial 30 4096 115200 115200 0 19200

vtx_halfduplex = ON
Allowed values: OFF, ON

# I've tried both options here:
vtx_smartaudio_early_akk_workaround = OFF
Allowed values: OFF, ON

# I've tried both options here:
vtx_smartaudio_alternate_softserial_method = ON
Allowed values: OFF, ON

# I've tried both options here:
vtx_softserial_shortstop = ON
Allowed values: OFF, ON

# I've tried both options here:
vtx_smartaudio_stopbits = 1
Allowed range: 1 - 2

vtx_band = 4
Allowed range: 0 - 5

vtx_channel = 4
Allowed range: 1 - 8

vtx_power = 4
Allowed range: 1 - 5

vtx_low_power_disarm = ON
Allowed values: OFF, ON, UNTIL_FIRST_ARM

vtx_pit_mode_chan = 4
Allowed range: 1 - 8

vtx_max_power_override = 0
Allowed range: 0 - 10000

vtx_frequency_group = FREQUENCYGROUP_5G8
Allowed values: FREQUENCYGROUP_5G8, FREQUENCYGROUP_2G4, FREQUENCYGROUP_1G3

• FC Board name and vendor: Matek F411 WTE (it has 2 UARTs and 2 softserial ports, the UARTS are used for GPS and CRSF and softserial 1 tx is used for IRC Tramp)
• VTX: Eachine Nano v3
• INAV version string: INAV/MATEKF411TE 5.1.0 Aug 19 2022 / 12:19:47 (76f22b25)
• I haven't tried an older INAV version.