Zero padding issue - cut the buffer
The result you experience is reasonable. The IDFT performs exactly the inverse operation of the forward DFT. Hence, for every signal $x[n], n=0...L$ you have the identity:

$$x[n] = \text{IDFT}\{\text{DFT}\{x[n]\}\}$$

This property is valid for DFT and hence also for FFT (as it is a special implementation of DFT). 

So, if you insert a zero-padded sequence $x[n]$ into the FFT and immediately perform IFFT you obtain the zeropadded sequence as the output.

The question is: Why would you want to do that? If you think of performing filtering in the frequency domain, dont do it.