Muxing 5.1 PCM places center channel hard left
After muxing from Blu-ray PCM, the 5.1 mix seems altered/flagged such that all center channel audio (read: dialog) is hard left/front in the image rather than centered in front. What's even more interesting is that demuxing the track with both mkvextract and ffmpeg restores the center channel to the center of the image.
Audio info from original .m2ts container:
Audio #1
ID : 4352 (0x1100)
Menu ID : 1 (0x1)
Format : PCM
Format settings : Big / Signed
Muxing mode : Blu-ray
Codec ID : 128
Duration : 15 s 940 ms
Bit rate mode : Constant
Bit rate : 6 912 kb/s
Channel(s) : 6 channels
Channel layout : L R C Ls Rs LFE
Sampling rate : 48.0 kHz
Bit depth : 24 bits
Delay relative to video : -1 ms
Audio info once muxed to MKV using MKVToolNix GUI v41.0.0 x64:
Audio #1
ID : 2
Format : PCM
Format settings : Little / Signed
Codec ID : A_PCM/INT/LIT
Duration : 2 h 23 min
Bit rate mode : Constant
Bit rate : 6 912 kb/s
Channel(s) : 6 channels
Sampling rate : 48.0 kHz
Frame rate : 200.000 FPS (240 SPF)
Bit depth : 24 bits
Delay relative to video : -1 ms
Audio info of standalone WAVE64 file (de-muxed using mkvextract):
Audio
Format : PCM
Format settings : Little / Signed
Codec ID : 1
Duration : 2 h 23 min
Bit rate mode : Constant
Bit rate : 6 912 kb/s
Channel(s) : 6 channels
Sampling rate : 48.0 kHz
Bit depth : 24 bits
I've also tried re-muxing the MKV-extracted .w64 file, which produces the same issue. Extracting the WAVE64 directly from the .m2ts then muxing again produces the issue.
I therefore have to assume that the issue lies with how MKVmerge is muxing 6ch WAVE. (And may also extend to higher channel counts, I just don't have anything higher than 5.1 in PCM.)