Sudden drop in comfort noise for fx enc at 24.4kbps stereo DTX

Basic info

Bug description

flt_enc -> flt_dec:

Screenshot_2025-06-20_at_17.32.17

fx_enc -> flt_dec:

Screenshot_2025-06-20_at_17.32.20

Above spectrogram shows a drop in the generated comfort noise when using the fx encoder. This indicates that the payload of the SID frame might already be incorrect. The float version also has a drop of one frame. This is also not correct and should not be mimicked by the BASOP code.

Ways to reproduce

./IVAS_cod_fx -stereo -dtx 24400 32 ltv32_STEREO.wav bit_fx
./IVAS_dec_flt stereo 32 bit_fx out_fx.wav