Thanks @Antun_Skuric for this explaination.
It’s clear now why the current is dropping while the BEMF (velocity) increases.
BTW, wouldn’t it be an idea to increase the motor.votage.limit to balance this electronics normal behavior ?
I mean a kind of proportionnal increase of this value with the velocity (in open loop mode of course)
current = (motor.voltage.limit * (1+ k.velocity) - Bemf)/R = (voltage + k.velocity - k.velocity)/ R
I will have to wait 2 weeks before getting the mag sensor. But for sure I will do a new video.
Regarding the heatsink, it is indeed very efficient (altough not the best place for a heatsink…). I can now sustain 2A into the coils without burning my fingers 
I also have a big copper pad under the chip which is also getting really hot.
And I could as well add a fan on this dirty CPU heatsink !
Right now, before getting the mag sensor, I will try to add an IMU to this and re activate my gimbal which has been sleeping since 2013…
JP