Right, but HDMI has no support for bitstreaming MLP, either. The only losslessly compressed surround formats it supports are the various flavors of Dolby, DTS, Auro-3D, and DST (compressed DSD). Otherwise, surround can be sent as uncompressed LPCM, DSD, or DoP (DSD-over-PCM). As expected, my Marantz SR6014 reports PCM input from my Oppo BDP-103D playing back DVD-A MLP 5.1. It would be the same with MLP 4.0, getting decoded to LPCM in the player and sent over HDMI uncompressed. This is why you have issues with your AVR and MLP 4.0, because it is sent no differently from any other 4.0 PCM signal (FLAC, Blu-ray LPCM, etc.).
By contrast, any AVR that can decode any flavor of Dolby or DTS will be able to map correctly the channels of 4.0 encoded in those formats, thanks to the metadata they contain. This includes the Quadio releases using DTS-HD Master Audio 4.0, which is why they play back correctly on your AVR.
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