The former, seems the handshake between the source and ATV4K to identify what type of content is being offered is on some content taking time. The ATV4K then accepts the content and it plays. If this involves an output change then I get the dynamic flash on my Sony Android whilst that resynchs. Messy and I feel that, taking iPlayer content as an example, that the output appears slightly softer than it was with tvOS 11.1 too.
Ah - I thought the flagging of frame rate and dynamic range needed to be flagged externally by metadata, not by decoding the video and trying to work out what is going on inside it from the file or GOP headers (so it doesn't require DRM decode for the initial phase).
Wonder what the change in perceived resolution is about - I guess if you aren't watching at 720p then there will be a scaling process and they may have changed that as well?
There's some chatter that some apps have now stopped working too, although I've not come across any. Presumably the initial handshake is failing to respond correctly.
The changes only work if you use Apple's internal video player app rather than implementing your own - unless you've been able to rewrite your own player (I thin). It's also optional (i.e. you can inhibit dynamic range / frame rate changes if your video is running in a window etc.)