Why is the last value of an RFFT always real?
The real component of an FFT or DFT represents cosine or even sinusoids (symmetric around the middle), and the imaginary component represents sine, or odd sinusoids (anti-symmetric around the center).  An anti-symmetric waveform has to be zero at the center of an even length window.  If a sinusoid of frequency Fs/2 is zero at any sample, it is zero at all samples taken at a sample rate of Fs.  The imaginary component of the basis vector at N/2 for a DFT transform is also all zeros for the same reason. Thus the imaginary bin at Fs/2 is the sum of zero times zero, and has to be zero.  Leaving the result in bin N/2 strictly real for even length DFTs.  Perhaps this is the bin your RFFT implementation reports for A(-1).

The is also the reason why Nyquist sampling has to be at a rate ABOVE the_highest_frequency / 2, not equal to.  Sampling will leave out part of any anti-symmetric waveform or waveform component (in any even length set of samples) if that component is of exactly frequency F when sampled at F/2.

There are different rules for odd length FFTs.