Audio out constantly switching between TV speakers and HDMI out. PUS9006

Hey there. So I just got myself a PUS9006 (70' if it makes any difference) and tried connecting my soundboard via HDMI Arc. Spent 2 hours trying to make the sound come out of it and when I finally did - the TV is constantly (sometimes once a minute, sometimes once every 5 sec) switching between audio outputs. It goes to TV speakers, then back to Audio system. Naturally during this time the audio is interrupted and it's super annoying. I'm using an ARC box for the soundbar, and I had a chance to run the audio output via the headphone jack while the arc box itself is connected via hdmi. However, sound quality in this setup is not the best and I would prefer it to run as it was designed to. Anyone else faced these issues? For reference, I tried all hdmi outputs, both arc and earc, reset the firmware, updated to the latest, but so far no luck.