This format does not exist anywhere else. It’s too long for a CD, the band does not know about this, the bonus tracks are from BBC sessions that were never released.
It’s like they create their own playlist and put it up for sale.
Does that make a proper release?
Do Amazon do stuff like this more often?
What do you mean with own format? As this is a download / streaming release the format should be “digital media”.
The digital release with the BBC bonus tracks is also available elsewhere, e.g. https://open.spotify.com/album/6wYMvax9kcq5U9sJmjOErQ, https://music.apple.com/de/album/sentinel-bonus-tracks/692350707 or https://www.deezer.com/de/album/316114
Apparently they must know about it as it’s an official release that is available across multiple services. It’s a digital release, not a CD one.
A ‘proper’ release is anything that’s sold/available to listen to, including for download. It doesn’t have to be available on a CD. Presuming that Parlophone holds the rights, it’s not a bootleg?
The ‘format’ comments by outsidecontext and tigerman325 are because ‘format’ in MB means a very specific thing - CD, digital media, LP etc. Amazon has not created its own ‘format’, it’s a regular digital media ‘format’ 
Thanks for the explanations. I understand about the formats on MB, I was just wondering whether it was something Amazon came up with themselves and whether that would warrant an entry on MB. But since there is this copyright notice (Parlophone) and it exists on multiple platforms.
I’m still going to find out more about this. The band don’t know about this, Parlophone don’t own the rights so must have asked for a license…