Z channel content potentially missing for FOA

During testing of the processing scripts, unexpected output was encountered when coding a test signal with the IVAS codec.

The input vector SBA_concatenated (please note 32kHz sampling rate)

After coding with FOA@160kbps passthrough, the output looks like this (commit 02c53074):

A similar behaviour was observed with the following noise burst vector too: pink_noise_4ch_48kHz.

Using a much longer diffuse noise vector does not result in the same behaviour:pink_noise_diffuse_N422_HOA3_norm.

I think we should verify whether this behaviour is expected or not, possibly the SBA focused devs might be the best to check?