I think your observations were / are incomplete. The behaviour described below dates back to at least April 2018 when I started looking at it; and hasn't changed to now with version 12.0.1 on my ATV4K (UK).
You can find evidence at the moment of DV titles in the Netflix library which behave differently to each other. Even same title, different season.
Take for instance Daredevil Season 2 episode 13, and then Season 3 episode 1. Details below; but the new S03 has fixed maxCLL at 4000, S01 and S02 both have wildly fluctuating maxCLL.
This is a problem for devices like the Lumagen which expect the content and / or player to be well mannered. The Lumagen for example uses this metadata change the tone map used; switching tone map isn't completely seamless and results in a luminance jump (you can see the tone map changed in the screenshots below). This shouldn't be an issue as these changes shouldn't be happening within content, they should only be happening at new stream boundaries.
Seeing as we have here two pieces of content, both DV, same player app, but behaving differently, I can see two possibilities:
1) The content has this fluctuating metadata burnt into the stream (either in error or by design of DV) and the AppleTV should be replacing this metadata with some static metadata and isn't, or the content provider should be making the content without the fluctuating data present. ATV DV is Profile 5 which has no native compatibility with HDR10, but I don't have details of what Dolby say should be happening with MaxCLL.
2) The AppleTV is actually creating the metadata from scratch and the DV streams are in some way differently mastered so some content causes bogus metadata to be generated by the ATV and some doesn't.
I don't have any way of instrumenting a test to see what is actually going on. The AppleTV HUD doesn't give enough useful information about what is happening in the streams and conversion process.
I've seen a couple of posts saying this MaxCLL data isn't bogus. It is
bogus - MaxCLL is defined as being a static value representing the maximum pixel value for the whole stream. If that metadata is changing during the stream it is by definition broken and bogus as it doesn't represent what that metadata is supposed to represent!
Details and screenshots
Season 2 Episode 13 is DV. Converted by the ATV to HDR10 it has wildly fluctuating maxCLL metadata, per your second point. MaxCLL should be constant for the duration of the program - by definition it is the max light level of a single pixel for the whole programme.
See a couple of grabs from the same episode S02 E13:
Season 3 Epsiode 1 is also DV. Converted by the ATV to HDR10 it has fixed maxCLL metadata: