@daniel here the setup is as follows:
The media is uploaded to <s3.eu-central-000.backblazeb2.com> as endpoint, but the instance bucket is private.
I'm using Bunny CDN, which authenticates with B2, downloads the media from <BUCKET-NAME.s3.eu-central-000.backblazeb2.com> and then shows it to the user.
So I guess that's why I didn't see this error so far. I'm already using the URL with the bucket name.
But anyway, good thing that you found the problem and fixed it in just one hour.
Cheers!