2020 QLED T - Low sound volume via soundbar from built in apps

[Charltones]

I have a Q90T and a Yamaha YSP-2700 soundbar connected via HDMI ARC (not e-ARC).

Sound from external devices (PS4, Wii-U, TiVO) is 'as normal' with the soundbar - i.e. anything over 50% volume is deafening.

Sound from certain built-in applications - Netflix and Disney+ for instance - is very low indeed. Some titles I've turned up and reached 100% volume and _still_ barely been able to hear what is being said. I've never had this issue before with the soundbar. When this happens and I press home on the remote to navigate the TV settings menu the TV sounds are super loud!

There doesn't appear to be any setting I can adjust that affects this. The audio settings on the TV are set to 'output format: Auto' and 'auto volume: Off'