Sudden drop in comfort noise for fx enc at 24.4kbps stereo DTX
# Basic info
<!--- Add commit SHA used to reproduce-->
- Encoder and Decoder (float): db364042fba738d5d0f9823c047e75edea62bae7
- Encoder (fixed): d3f2d47ab8c8135bc56a7c5eb27ed7e8997f210d
# Bug description
flt_enc -> flt_dec:

fx_enc -> flt_dec:

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
<!--Commandline or script-->
```bash
./IVAS_cod_fx -stereo -dtx 24400 32 ltv32_STEREO.wav bit_fx
./IVAS_dec_flt stereo 32 bit_fx out_fx.wav
```
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issue