Artifact in IGF part for first active frame after DTX period in MDCT-Stereo

Basic info

• Fixed point: f99c9ea3 (latest main)

Bug description

At the first actively coded frame after a DTX period, there is an artifact occurring in the BWE part of the spectrum at. This is a follow-up from #1311 (closed)..

See artifact in the spectrogram at 13.5s:

Ways to reproduce

Box folder: ...\Box_EXTERNAL_IVAS_BASOP_VERIFICATION\issues\issue-1311

./IVAS_cod_fx -q -dtx -stereo -max_band FB 80000 48 stereo-noisy_speech-p800-2-cat1-lab_b-20ms_aligned_files_concatenated_48.wav out/stereo-noisy-speech.stereo-noisy_speech-p800-2-cat1-lab_b-20ms_aligned_files_concatenated[0dB].enc[48kHz-FB-80000-dtx-fx].192
./IVAS_dec_flt_ref -q stereo 48 out/stereo-noisy-speech.stereo-noisy_speech-p800-2-cat1-lab_b-20ms_aligned_files_concatenated[0dB].enc[48kHz-FB-80000-dtx-fx].192 out/stereo-noisy-speech.stereo-noisy_speech-p800-2-cat1-lab_b-20ms_aligned_files_concatenated[0dB].enc[48kHz-FB-80000-dtx-fx].dec[clean-48kHz->stereo-flt].wav