I have just upgraded one of my sites, not the Prod site, the Local one. using a script I have created which contains the command below:
drush dlb backdrop --2020-05-21 17:45:47-- https://github.com/backdrop/backdrop/releases/download/1.15.3/backdrop.zip
As you can see it downloads version 1.15.3 which is the PREVIOUS Version!!
Downloading from the Link in the Available Updates Page all is well, so the link using Drush must broken.
When I upgraded to 1.16.0 a week or so ago using my script I had no problems.
Regards to the Team.
@themetman - I don't have a good answer for you, but I can at least provide some content. We've only recently started to provide preview releases of minor releases (in this case 1.18).
This appears to be a bug in Drush, not anticipating preview releases and knowing that it should not recommend them.
I suspect, that this will no longer be an issue, once the official release for 1.18 is out in 9 days.
This does not solve your problem today, but hopefully puts it into context a bit.
I opened an issue in the drush issue queue. I regret to say that you may need to manually update Backdrop until the next version comes out on Jan 15th.
https://github.com/backdrop-contrib/backdrop-drush-extension/issues/236