Bug finding subgroups of the class group?

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This is potentially a bug, but I want to check whether I'm just doing something wrong here:

When I try to find the subgroups of the class group of a number field, I am often (though not always) getting a list of only the trivial subgroup repeated many times. Here is some example code:

This first example works as expected:

sage: K.<a> = NumberField(x^2 + 191)
sage: clG = K.class_group()
sage: clG
Class group of order 13 with structure C13 of Number Field in a with defining polynomial x^2 + 191
sage: clG.subgroups()
[Multiplicative Abelian subgroup isomorphic to C13 generated by {Fractional ideal class (2, 1/2*a + 1/2)},
 Trivial Abelian subgroup]

This second example, however, does not:

sage: K.<a> = NumberField(x^2 + 21)
sage: clG = K.class_group()
sage: clG
Class group of order 4 with structure C2 x C2 of Number Field in a with defining polynomial x^2 + 21
sage: clG.subgroups()
[Trivial Abelian subgroup,
 Trivial Abelian subgroup,
 Trivial Abelian subgroup,
 Trivial Abelian subgroup,
 Trivial Abelian subgroup]

It looks to me like this bug is occurring whenever the class group is not cyclic. Is this a known issue, or is this something I should report? And if so where/how do I report these bugs (I haven't done any development before)?