I think you might be right looking at it again. Probably explains why its sluggish to climb but drops abruptly when descending. This is when hover throttle is < 1500. The opposite is probably true with hover throttle > 1500.

Well, yes, that's why I started to figure it out #10547. I've only just figured out how to fix it completely.

It seems that fixedwing also suffers from this error
https://github.com/iNavFlight/inav/blob/master/src/main/navigation/navigation_fixedwing.c#L112

Your solution above isn't right because it'll always be adjusting if there's no deadband applied, which you don't want. The issue is the way the adjustment is converted into a climb rate. Would need to check the numbers but it doesn't appear that max throttle gives you max_manual_climb_rate and min throttle -max_manual_climb_rate ... you get something else.

Fixed wing is probably OK because it doesn't use a variable altHoldThrottleRCZero factor.

Actually checking this with debugs shows that the actual climb rate is just a bit higher than it should be but is consistent up and down. e.g. max_manual_climb_rate set to 700 actually results in +770 up and -770 down at max throttle stick deflections with altHoldThrottleRCZero set to 1320. So not exactly correct but not different up and down as I thought originally.

Yes, it is. But I don't understand anything again. Why is the tgtVel correct in this case?
The deaband was applied somewhere else. Need more time to check

I found my mistake. I restricted max climb rate to 1m/s.

I guess that correct math will be like this
`
const int16_t rcThrottleAdjustment = rcCommand[THROTTLE] - altHoldThrottleRCZero;

    if ((abs(rcThrottleAdjustment)>rcControlsConfig()->alt_hold_deadband)) {
        // set velocity proportional to stick movement
        float rcClimbRate;

        // Make sure we can satisfy max_manual_climb_rate in both up and down directions
        if (rcThrottleAdjustment > 0) {
            // Scaling from altHoldThrottleRCZero to maxthrottle
            rcClimbRate = rcThrottleAdjustment * navConfig()->mc.max_manual_climb_rate / (float)(getMaxThrottle() - altHoldThrottleRCZero);
        }
        else {
            // Scaling from minthrottle to altHoldThrottleRCZero
            rcClimbRate = rcThrottleAdjustment * navConfig()->mc.max_manual_climb_rate / (float)(altHoldThrottleRCZero - getThrottleIdleValue());
        }

`

But. This is incorrect too, because velocity starts not from zero.
May be this will be correct:
const int16_t rcThrottleAdjustment = applyDeadbandRescaled(rcCommand[THROTTLE] - altHoldThrottleRCZero, rcControlsConfig()->alt_hold_deadband, -500+getThrottleIdleValue(), 500);

Although even so, perhaps
const int16_t rcThrottleAdjustment = applyDeadbandRescaled(rcCommand[THROTTLE] - altHoldThrottleRCZero, rcControlsConfig()->alt_hold_deadband, getThrottleIdleValue()-altHoldThrottleRCZero, getMaxThrottle() - altHoldThrottleRCZero);

I have a fix for this. I'll do a PR when I have time.

Ok. I already fix this for myself.
On my setup difference in descent vel was more then three times.
max_manual_climb_rate was about 150

Fix this too, please, because errors tend to accumulate, and after some time everything stops working.

Max value of rcAdjustment is 500, but not 500-deadband.

It works fine, but I changed code a little for clarity
int16_t rcClimbRate = rcThrottleAdjustment * navConfig()->mc.max_manual_climb_rate / ABS(limitValue);