Bitrate switching artefact when going from STEREO DFT to MDCT

with 94e7768c and this bitstream

ltv_STEREO_32k_-stereo_sw_13k2_to_128k_10fr.bin_32.bit.zip

We can see important artefact when going from a low bitrate to a higher one. It is not systematic, but when it happens it seems to always be from DFT to MDCT and the artefact is an ear blowing kind of artefact.

The figure below shows (zoom between 94s-99s) the visual effect on the left channel: From top to bottom:

element bitrate
element mode
left channel float
left channel fixed-point