Uncontrolled descend after octo motor failure


I’ve got octocopter descend due to non-apm related reason during auto-tune. Motor #3 wires soldering was bad, it stopped spinning in the air and octo falled down.
Same airframe with Naza V2 still continues to fly even if two motor failed.

I would like to ask community advise wich settings could be tuned to leave octo in hover with alteast one motor failure.

Octo configuration:
17x5.4 T-Motor props, 5008 340kv, 5S, 8.5kg take-off weight, 0.65 weight-to-power ratio
Pixhawk PX4FMU 2.4

APM logs attached.

My assumption what some AltHold related settings should be adjusted.

At the time then IMU detected motor #3 not responds as expected it send full throttle CH3 RCOUT PWM (1918us) as well as CH1 and CH8 full throttle (1918us too):

Record 53114 (19:08:48.302) - full throttle to CH3
Record 53194 (19:08:48.702) - ThrOut set to 1000 (max), IMU altitude falls down
Record 53482 (19:08:49.964) - CH1, CH3, CH8 throttle set to 90-100%. CH3 physically not responds but neighboring motors CH1 and CH8 helps to stabilize angles, IMU altitude goes up.
Record 53722 (19:08:50.998) - CH3 max, CH1 and CH8 falls down below hover level required to fly with 7 motors.
Record 53805 (19:08:51.395) - IMU altitude start to falls down again. ThrOut max, CH1-CH8 RCOUT (except CH3) from 1172 (CH7) to ~1800 (CH4), CH3 max (1918).
Record 54314 (19:08:53.675) - Desired altitude goes down, IMU can’t hold required altitude.

During all this time ThrIn was 50% or above.

I would be very glad if anyone can make advise for following questions:

Are there any settings which forces IMU to set higher level of RC OUT channels in AltHold if ThrOut set to 1000 in case then 1 motor fails?
Does it makes sense to take-off in Stabilize mode with 7 motors only to be sure that this is AltHold settings issue?