What is the significance of a negative imaginary component in a S21 measurement ?

For example, in the following plot of a passive network, the linear magnitude of the S21 parameter decreases with frequency monotonically. IOW: It acts as a low-pass filter.

However, the imaginary component is not monotonic - it has a minimum at 1.56MHz and is always negative (below the Ref 0U). What does that negative sign signify ?

If this were an Impedance plot (it is not), then the negative sign would signify capacitive reactance.

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