LE12: Always in HDR mode (HDR at boot)

When I connect my Intel NUC direct to my 4K Sony TV the display is always in HDR mode (HDR always on). It never switches off.

When same NUC is connected via my AVR to TV all runs as expected, HDR is only activated when I play HDR media and off when playback stopped.

How can I solve the direct connection ‘HDR always on’ issue? (I want to use ARC to my AVR)

I’m running ‘standard’ LE12 nightlies, I first noticed this a couple weeks ago when swapping NUCs around. I’m currently running nightly from 31 March.

It may not be a LE issue as when the NUC is switched on the Intel NUC boot splash appears to be in HDR mode. But this issue only occurs when it’s directly connected to the TV, never when it’s connected via my AVR.