I have the exact same result. Although my LG Tv is not supposed to be HDR10+, fact is the native player of the Tv does wonders with the bright colours from this video, the HDR icon comes up and it is really nice. Couldn't get that result with the U22-XJ, it apparently didn't even recognize it as HDR.
But it will work perfectly on the U22 if you try to read it with Kodi on CoreElec, so I guess it has something to do with Android and not the box itself.
The fact that I need to set SDR to HDR on in order to have the file played as static HDR (otherwise it'll look washed out) hints that the box is not even considering it as HDR / HDR10+ by default. The SDR to HDR option seems to attempt a tone mapping when reading the file, which works pretty well as the file is already in 10bits with the full colour spectrum needed for HDR. However as the box believes it is not a native HDR file, all the HDR10+ capability goes away as it's considered SDR rendered in HDR.