Microphone Array null steering unexpected results after IFFT
I don't understand what you did but I think you didn't understand the idea correctly.
Lets say the weighting vector at a certain frequency is given by $\mathbf{w}^H$ where ${}^H$ is the complex conjugate. That is the elements of $\mathbf{w}^H$ are the weights you apply to each sensor without null steering. And lets say that the array response to the interference is $\mathbf{v}_I$ (which is basically the conjugate of the array weights if you were to point the beamformer to the interference). The simple null steering solves the following optimisation problem:
$$   \underset{\mathbf{w}_{d}}{min}\:\left\Vert \mathbf{w}_{d}-\mathbf{w}\right\Vert ^{2}\\
  subject\; to\\
  \qquad\mathbf{w}_{d}^{H}\mathbf{v}_{I}=0 
$$

Using Lagrange multipliers it can be shown that
$$ \mathbf{w}_{d}^{H}=\mathbf{w}^{H}\cdot\left(\mathbf{I}_{N}-\frac{1}{N}\mathbf{v}_{I}\mathbf{v}_{I}^{H}\right)$$
where $\mathbf{I}_{N}$ is the identity matrix and $N$ is the number of sensors.

This solution is not very robust to errors in the interference direction.