I can't promise it will work or help. I'm not even an Apple user but I've been fighting with DolbyVision and trying to generate anything resembling a functional HDR10 file in any single piece of software for the past month so I feel your pain. Final Cut Pro apparently doesn't do anything with the Dolby 8.4 metadata and can't convert it to level 8.1, just output it... as 8.4 alongside an HLG file converted to PQ, which it won't work with. Thanks? (
https://www.apple.com/final-cut-pro/docs/HDR_WideColor.pdf)
You might try:
https://github.com/quietvoid/dovi_tool, which can extract Dolby Metadata from a video stream, and convert it to profile 8.1 which works on all LG TVs. Whether this works on 8.4 metadata is anyone's guess, it wasn't designed for that really. Maybe it'll be added directly. That's why I'm leaving the link here. It might just be a matter of changing the compatibility level from 4 to 1 in the NALs.
As I understand it, DV metadata is mostly the same between the various compatibility levels (the 4 in 8.4 is HLG w/ BT.2100 gamut in BT.2020, 3 is BT.709 in BT.2020, 2 is SDR BT.709, 1 is HDR10) as it (mostly) represents transforms that should be applied after and during conversion to the HDR color space in the display.
Prior to these phones there was no profile using compatibility level 4. TBH this being HLG and many news agencies having fired their expensive photographers and handed their reporters iPhones to shoot grainy, blurred vertical video with for the 9:00 news, I wouldn't be surprised if the entire reason for inclusion was just a few major news outlets asking Apple for it. They get to sell it as a feature to the general public (even though it doesn't actually *work* with anything) but it's not really for you... there's no technical reason they couldn't have just generated 10bpc SDR + DV 8.2 instead. Most people are just going to be clipping and sharing video, and everyone who can play DV to begin with could have played that.
I guess there's the off chance that someone is insane enough to want to do color / HDR grading on their 10bpc 4:2:0 HLG cell phone video, but then the dolby metadata is pointless. Before somebody says it, compatibility level 4 requires 4:2:0 chroma subsampling so outputting the video as something better for grading would break it. Outputting a higher bit depth isn't supported by any DV level either. HDR10+ supports greater bit depths / chroma but it doesn't exactly have a ton of support and for being an open standard I can't find the specs for it anywhere.
I thought it might be some kind of backwards compatibility for the non-HDR (certified) computer displays most people use, but then making the DV part incompatible with basically everything that can see it seems kinda useless.
Anyway like I said, not an Apple person so I won't be sticking around, just thought I'd hopefully help a little.