IO and FMU PWM had different states of prearming during initial ramp up

[MaEtUgR]

Describe the bug
When I mixed the exact same outputs to IO and FMU on a Pixhawk 4 and looked at the logged outputs they differed during the PWM ramp after arming. The IO skipped the PWM ramp and directly went to idle because it was prearmed during the ramp since prearm overrides any ramp: https://github.com/PX4/Firmware/blob/master/src/lib/output_limit/output_limit.cpp#L119-L121 The FMU did everything correctly as expected and was not prearmed during the ramp.

FMU prearm flag condition:
https://github.com/PX4/Firmware/blob/master/src/lib/mixer_module/mixer_module.hpp#L186-L189
IO prearm flag condition:
https://github.com/PX4/Firmware/blob/master/src/modules/px4iofirmware/mixer.cpp#L213-L220
They are not exactly the same and I didn't understand what exactly the difference.

I would have fixed it already if I'd understand the conditions and where the data originates for the IO.

JFYI @cmic0

[taileron]

Possibly also the cause for #15155 in the tiltrotor with activated IO switches mot 3 off as it should be in FW. In Acro mode it also switches off but there is another bug that mistakenly mixes MC pitch in FW to mot 1 and 2.

Without IO, FMU behaves completely wrong and just lets Mot 3 continue to run in all flight modes (that also happens with only FMU FCs e.g. PixRacer).

[stale]

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