Missing energy on stereo CNG frames at 24k4 kbps

The energy above 2kHz seems to be missing or has been attenuated for CNG frames.

commit id: e6911975

To reproduce run:

IVAS_cod.exe -STEREO -DTX 24400 48 stvST48n.wav bit
IVAS_dec.exe STEREO 48 bit out_stereo_24k4_dtx.wav

The test signal can be downloaded from: https://forge.3gpp.org/rep/ivas-codec-pc/ivas-codec/-/blob/main/scripts/testv/stvST48n.wav