Dolby vision content intermittently dimming, fluctuating brightness
I've been using an Ugoos AM6B+ for about a year connected straight into my LG G4 without any issue, it's worked perfectly, but I recently got an Onkyo TX-RZ30 receiver and I noticed that when I watch dolby vision content the brightness fluctuates. It's most noticeable in bright scenes, it will randomly dim (maybe 20%) and stay like that until the scene changes.
The settings on the TV and the Ugoos are exactly the same, all that's changed is it's now plugged into the receiver with the same HDMI cable. SDR and HDR10 content are fine. A dolby vision movie played through my blu-ray player is fine, and for sanity's sake I plugged that cable into the ugoos with no improvement, so it's definitely something to do with the ugoos. I know this is a coreelec subreddit and not a receiver subreddit but I thought I'd be best asking here. Does anyone know why this might be happening? It's driving me mad.