One channel disappears in the EVS compatible stereo downmix

Bug description

When checking operation of the EVS compatible stereo downmix, especially at lower sampling frequency rates (e.g., 16 kHz) there is a quality issue with one channel that is too strongly attenuated and becomes inaudible in the mono downmix output. This issue was found when validating fixed-point with extra test cases, it is also present in the reference floating-point.

Ways to reproduce

IVAS_cod -stereo_dmx_evs 24400 16 a1if1s2_a1im1s3_16.wav a1if1s2_a1im1s3_16.bit 
IVAS_dec 16 a1if1s2_a1im1s3_16.bit a1if1s2_a1im1s3_16_out.wav