AC measurement in a circuit
Well, it isn't true that AC simulation only returns positive values.

The input to an AC analysis is a sine wave with a given amplitude and phase. The output again is a sine wave with an amplitude and phase determined by the transfer function of the system.

So the result consists of two values which are usually computed as a complex number. The representation often consists of the magnitude and the phase, since they are easier to work with than the real and imaginary part.

In order to see if a value is negative, it is therefore also required to check the phase. For a negative result an additional phase shift (relative to the input signal) of 180 degrees should occur.