[Python] Understanding weird boolean 2d-array indexing behavior in numpy

Why does this work:

a = np.random.rand(10, 20)
x_range = np.arange(10)
y_range = np.arange(20)

a_tmp = a[x_range<5,:]
b = a_tmp[:, np.in1d(y_range, [3,4,8])]


and this does not:

a = np.random.rand(10,20)
x_range = np.arange(10)
y_range = np.arange(20)

b = a[x_range<5, np.in1d(y_range,[3,4,8])]

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