IFFT and sample count issue in OFDM
First value is from cyclic prefix and last value is the last sub-carrier which you are messing up by windowing of 1 sample. As a result of this, the frequency orthogonality of the sub-carriers is lost. And this inter-carrier interference is causing your QAM sliced symbols to randomly change phase and amplitude around true value.

The way actually windowing is done is by adding cyclic suffix of one sample in the end also like cyclic prefix and then overlap the next symbol with the first one by 1 sample. Then ICI won't happen. For a detailed code of how windowed overlap and add (wola) is performed you can check my code at : 5GNR Wola Waveform Simulation