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The issue lies in the power 3/2, which requires canonicalize_radical to be simplified. This method is not included in simplify_full because it has some arbitrariness in the choice of square roots.

First of all, you should not perform the double substitution in eq1, but keep only the first one:

sage: eq=eq1.subs({y:sqrt(-x^3+(121/3)*x+(1690/27))})

Then you are done:

sage: eq2=eq.canonicalize_radical(); eq2
0

For more details on canonicalize_radical, type eq.canonicalize_radical?.