BASOP Stereo encoder at 13.2kbps is sometimes missing energy in subframes

Basic info

• Encoder and Decoder (float): db364042
• Encoder (fixed): 574a190e

Bug description

In this selection test stereo item coded at 13.2kbps, the BASOP-encoded output has some (sub)frames with missing energy, for example at about 5.0s:

float:

BASOP:

And at 4.11s, where mainly the high band seems to be affected:

float:

BASOP:

And at 3.46s:

float:

BASOP:

Those issues are not very audible as the general quality at this bitrate and with this challenging item is already quite low. Nevertheless, it would be good to understand where those drops in energy come from.

Ways to reproduce

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

./IVAS_cod_flt_ref -q -stereo -max_band SWB 13200 48 am1aa1s12_48.wav out/stereo-mixed-music.am1aa1s12[0dB].enc[48kHz-SWB-13200-flt].192
./IVAS_dec_flt_ref -q stereo 48 out/stereo-mixed-music.am1aa1s12[0dB].enc[48kHz-SWB-13200-flt].192 out/stereo-mixed-music.am1aa1s12[0dB].enc[48kHz-SWB-13200-flt].dec[clean-48kHz->stereo-flt].wav
./IVAS_cod_fx -q -stereo -max_band SWB 13200 48 .am1aa1s12_48.wav out/stereo-mixed-music.am1aa1s12[0dB].enc[48kHz-SWB-13200-fx].192
./IVAS_dec_flt_ref -q stereo 48 out/stereo-mixed-music.am1aa1s12[0dB].enc[48kHz-SWB-13200-fx].192 out/stereo-mixed-music.am1aa1s12[0dB].enc[48kHz-SWB-13200-fx].dec[clean-48kHz->stereo-flt].wav