about sampling frequency
The second plot is correct, 25 MHz tone where you halved your input frequency to 25 MHz. I suspect that you may be sampling synchronously a real signal such that the samples are all landing close to the zero crossings (noticing the dB scale on the horizontal axis on the first plot that there is minimum signal content in your FFT). If this was done with complex sampling the tone would always show up somewhere in the digital domain from 0 to $F_s$. But with real sampling we can get a null at $F_s/2$ as you are seeing with synchronous sampling at or close to the zero crossings.