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I can not reproduce your problem on 7.4.beta2:

sage: var('x' ,'y' ,'z', 'lam')
(x, y, z, lam)
sage: F = x*y*z - lam * (x*y*z-9) 
sage: F
x*y*z - (x*y*z - 9)*lam
sage: dFx=diff(F,x)
sage: dFy=diff(F,y)
sage: dFz=diff(F,z)
sage: dFlam=diff(F,lam)
sage: dFz
-lam*x*y + x*y
sage: solve([dFx==0,dFy==0,dFz==0,dFlam==0],x,y,z,lam)
[[x == 9/(r1*r2), y == r1, z == r2, lam == 1]]