I encounter exactly the same issue with a NUC6CAYH Apollo Lake NUC. I've also removed the internal Pulse Eight adapter and installed the external Pulse Eight USB adapter => same issue!Which gives me the idea that it has to do something with Libreelec + the newer NUC hardware since you confirmed you have no problem when using Kodi on Win10. I didn't try this yet myself.
Maybe we can test ubuntu + Kodi or Openelec to see if this setup has the same behavior?
Sounds like a plan
I will try to test this during the weekend.
So how is the soundbar connected?
Is the NUC connected to the soundbar or to the TV?
HDMI 1 (Ultra HD Deep Colour - activated) -> NUC
HDMI 2 (HDMI-CEC) -> Soundbar
HDMI 3 -> XBOX One
I've already connected the NUC to my Soundbar, but this do not solved the problem