Question: Can Maple find non-negative real solutions of this particular polynomial equation?

The algebraic equation in four variables is: 

expr := (a^2 + b^2 + c^2 + d^2 - 4)*(a^2 + b^2 + c^2 + d^2) + 4*d*c*b*a - (a + b + c + d - 2)*(a + b + c + d) + 6:
solve(expr = 0, useassumptions, allsolutions) assuming nonnegative;

Unfortunately, I've been waiting for a long time, and I have no more time to wait; I have to interrupt the current session by hand. However, it appears that MMA can solve it within bearable time:

Did Maple miss something here?

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