This might not be something you want to deal with, since it appears to be custom metadata, but I thought I might bring it up.
I came across this SuperUser question (https://superuser.com/questions/1438821/adding-custom-metadata-tags-to-mp4) where someone added custom metadata with the name "Projection Type" (space included). I followed the instructions there and created this test file (https://www.dropbox.com/s/wh6y3ofd2pbthmp/Exiftool_Forum_Metadata_With_Space.mp4?dl=0).
Exiftool doesn't show the custom "Projection Type" tag unless the -u option is used and then showed it as Unknown_0x0005. FFProbe listed the data as "Projection Type".
Ah. Thanks for the sample. I didn't know that space was a valid character in a Key name, and ExifTool was rejecting this. It will be fixed in the next version.
Also, I see that the group names for most of the Keys tags are wrong. (Reported incorrectly as ItemList.) I will fix this too. (Technically, Keys tags are in the ItemList, but they are stored differently because their names are taken from entries in a Keys directory, unlike other ItemList tags, so I put them in a different group.)
- Phil