Particular in precomp, camera linked to Main comp cam

Hi everyone ! :-)

I'm facing a problem I don't understand (yet !). Here it is :

I have 1 comp and 2 precomps.
- Main Comp : the 2 precomps + 1 main camera
- Precomp "Left" : 1 Particular "Left" + a camera (copy of the main one)
- Precomp "Right" : 1 Particular "Right" + a camera (copy of the main one)

In order to command all cameras from the main comp, I added these expressions on the precomps cameras :
comp("MainComp").activeCamera.pointOfInterest
comp("MainComp").activeCamera.position

But as soon as I did it, the Particulars were not visible anymore !
WHY ??
I checked, the expressions are working fine... So, why do the Particulars bother ?

A great thanks for your help ! :-)