Mathematica’s ToNumberField function doesn’t always work.

I’m seeing a problem with ToNumberField sometimes claiming that a
number isn’t in a number field, when it actually is.

In particular, try

d=Root[-5+17 #1^2-8 #1^4+#1^6&,6];
z=Sqrt[2-d^2];
f=Root[-5+413 #1-2156 #1^2+#1^3&,2,0];

now ToNumberField[f, z] fails, but ToNumberField[d^2 f,z]/ToNumberField[d^2,z] (which gives the same answer) succeeds.

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