Here's scenario A (sampling at the zero crossing point): -

[![enter image description here][1]][1]

Image from [here](https://dsp.stackexchange.com/questions/53616/nyquist-frequency-confusion).

Based on the samples taken, you'd estimate that there was no signal and that ties in with this: -

> *at the exact Nyquist frequency, 50 kHz, the amplitude is heavily
> reduced*

On the other hand, if the samples happened to be offset by 90° you'd conclude that the amplitude was correct.

> there seems to be a DC component

Your image shows nothing but, with some waveforms there might be or, it might just be a residual DC value.

> 
> *What is failing in the theoretical Nyquist theorem.*

Nothing as far as I can see. The theorem states that the sampling frequency has the be **greater than** the maximum signal frequency.


  [1]: https://i.sstatic.net/IMs55.png