camera.contrast increase at each new capture with overlay

[watchever]

Hello, I have set the contrast value of the camera for example at value=10 for a serie of captures.
Set awb_mode and exposure_mode to off.
Then I loop and take several captures and for each new capture the contrast increase... I guess it increases of the value I have initially set. I must set back the contrast to 0 to stop the increasing contrast.
It is like contrast is a relative value to the previous one...
Is it normal ?

My source code :

camera.iso = 800
camera.shutter_speed = 250000
camera.exposure_mode = 'off'
g = camera.awb_gains
camera.awb_mode = 'off'
camera.awb_gains=g
camera.contrast = 10

camera.capture(output, 'rgb') #output is a numpy array
overlay = camera.add_overlay(output.tobytes(), format='rgb', layer=3)
for i in range(10):
camera.capture(output, 'rgb')
overlay.update(output.tobytes())