With the above combination in the audio section if you turn on passthrough audio, the only options are DD,EAC-3 and DTS, no TrueHD. I tried the latest distro and it's the same but reverted back to 8.2.5 since the latest crashed and hung on my regularly.
Is this a Pi3 limitation or a limitation in LibreElec? Without TrueHD I can't get Atmos support.
Interesting if I play content in TrueHD (running off Plex) I can hear audio but I presume the Pi3 is playing the DD core only.