modular_symbols_of_sign(1) giving incorrect answer

[kevinywlui]

sage: A = J0(188)[0]
sage: M = A._modular_symbols()
sage: M
Modular Symbols subspace of dimension 8 of Modular Symbols space of
dimension 49 for Gamma_0(188) of weight 2 with sign 0 over Rational
Field
sage: M.modular_symbols_of_sign(1)
Modular Symbols subspace of dimension 12 of Modular Symbols space of
dimension 27 for Gamma_0(188) of weight 2 with sign 1 over Rational
Field

The dimension of

M.modular_symbols_of_sign(1)

should be 4 instead.

Component: modular forms

Keywords: modular symbols

Issue created by migration from https://trac.sagemath.org/ticket/20821