Real-valued ringing when zero-padding odd-length FFT
By zeroing out high frequency bins, you have effectively multiplied the spectrum of the signal with a rectangular function.  Multiplication in frequency is convolution in time and the Fourier pair of a rect is a sinc.  So what you really did was convolve the time domain signal with a sinc with the width of the main lobe of the sinc inversely proportional to length of the rect.  This is why the numerous filter designing techniques like Parks-McClellan design in what's called a "transition region" or "transition" band so that there is not an instantaneous change in the frequency response of the filter.  These filter design techniques are important because the "ideal" filter like you used has such undesirable effects in the time domain.