During throttle up, motor speed is dropping down after seconds of running

Hi. I’'m new here and I would like to ask on my issue with my drone below. My drone type is quadrator. I’m using PX4 version 5 for my Flight Controller and I’m monitoring my Flight Log using Mission Planner.

The issue is that, motors cannot maintain rotation at 1900-2000RPM range (ideal take off speed) during initial throttle up whereby after few seconds, motor 1, 2 and 3 RPM drop drastically more than 10% decrement.
It was observed that after 7-8 seconds running time, motors are slowing down and not maintaining at desired speed.

Motor 1: Speed reduction to 1622 from 1855 RPM (-12 %)
Motor 2: Speed reduction to 1702 RPM from 1903 RPM (-10.56 %)
Motor 3: Speed reduction to 1145 RPM from 1822 RPM (37.16 %)
Motor 4: Speed maintains at 1944 RPM from 1944 RPM (by right, this condition should be the ideal case).

I already checked on hardware and it seems that no issues at all , motor, ESC, FC, power distribution board etc. are ok based on the troubleshooting. I checked then on my Dataflash Log, I can see the signal from FC to ESC are not right. You can refer RCout parameter from my attachment for reference.
I just want to ask here, what could be the potential problem related to this unbalance signal to the ESC? as I can feel that something is not right with the signal to all 4 motors. Hope all get my points .TQ in advance.

Did you find any solution to this?

Could you please provide a log file? I’ll help you analyze it.

This may sound like a silly question, but has this drone ever flown correctly?

What has happened between it flying correctly and now its not flying.

Thank you. The drone was bench tested without propellers in stablized mode with full throttle. https://review.px4.io/plot_app?log=c3f45ee3-8c4c-4161-847a-3f818a0e3f17