Wrong factorization in QQ['x,y']

5.8 on ubuntu and on sagenb.org.

sage: Kxy.<x,y>=QQ[]
sage: P=-10*x^4*y + 40*x^3*y^2 - 80*x^2*y^3 + 80*x*y^4 - 30*y^5 + 20/9*y^3 + 
10/81*y
sage: fa=factor(P);fa
(-10/81) * y * (-3*x + 3*y + 1) * (-27*x^3 + 81*x^2*y - 135*x*y^2 + 81*y^3 - 
9*x^2 + 18*x*y - 27*y^2 - 3*x + 3*y - 1)
sage: factor(fa[-1][0])
(-3*x + 3*y - 1) * (9*x^2 - 18*x*y + 27*y^2 + 1)
sage: fa[-1][0]
-27*x^3 + 81*x^2*y - 135*x*y^2 + 81*y^3 - 9*x^2 + 18*x*y - 27*y^2 - 3*x + 3*y - 
1

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