I believe that kodi can access the hardware directly (ie kodi settings override any built in settings), so while it is good to know to confirm that the receiver is getting passthrough on one app, just need to narrow down what/why the shield isn’t passing through with plex.
if you want to get that far into, you could sign out of your account in the shield plex app, go into the shield > app settings, force stop, clear cache, clear data, that should give you essentially a clean plex install on the current version.
you may have to go back in, re-sign in, re-set any library pins/hubs, and set the plex settings to hdmi/etc, I don’t know what the defaults are. If you have a lot of customization, it might be painful.
when I get home, I’ll get screenshots of both my shield settings and plex settings. (that will be good for my own records too)
then a matter of stare and compare.
