Problem with full throttle motor stop pushing

Hi, I tried to be as specific as possible in the title.

This is my problem, I work for an small neighbor and we bought 10 e bikes from a quite professional builder at china.

When they sent the sample with bafang hub motor it worked great.

Then to make it a little cheaper we decided to use a cheaper motor that they recommend us.

The deal is next: The bike has pas sensor for pedal assist and also throttle, if you are stop, and you use full throttle from cero, the motor push at full power for a second and then it just stop pushing. I´m an electronic engineer, so I start debugging the problem.

1) I realized it just happen when the batteries where not full charged (some cases also with full charged battery), I check battery voltages at full charge and 5 more times while it was getting discharge time to time and it is correct comparing with the discharge curve.

Then, few times this problem happened, the KT LCD showed error 06, this error means a short circuit at the motor or controller. But the error desapegar immediately and the bike start working well again.

2)I realized that if we accelerate from cero but very cerfull to not use full throttle, the bike works well.
(I started to think it could be a problem of current limit at the motor or the controller)

3)I got into the KT LCD configuration, and at the C5 parameter (it limits the current at the controller) it was set at number 10, it means no current limit. So it would mean that if there is a current problem it should be at the motor.

So I set the C5 parameter to 01, it is slow start, so even if you push full throttle from cero, the bike will accelerate smoothly.

After this, the bike started to work well, so I believe I confirm it was a current issue, and the error 06 (short circuit) was given just because of the full throttle the controller mesure the current and felt it was too much. (perhaps because of the motor internal resistance was too little)

The strange thing is that the motor is 500w and it used to stop pushing when it gets very quickly the 650W..

From china they are telling me to test different things but I understand that it is a current limit problem, does any one had this problem or know what could be causing it?

Thank you very much from now if you answer me.