When using the WAVEFORMATEXTENSIBLE format, the GUID identifying the type is not preserved.
For an example, the files in
are in first-order Ambisonic B-Format, which has four channels (WXYZ), a special GUID identifying the format, and no channel mask (because the channels do not map to speaker positions).
The definition of the AMB format is at
More examples of AMB filetype can be downloaded from http://ambisonia.com/
Compressing these using flac -8 --channel-map=none: on decompress, the extensible type is reset to "rectangular quad" and speaker mask is set.
If the GUID is retained then the speaker mask will be ignored by Ambisonic decoders. Without the GUID the file is no longer a valid Ambisonic wav and cannot be decoded.