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Why are images from the Docker Hub prefixed with "docker.io/" on some platforms and not on others?

I’ve never seen this before recently, but on Fedora 26 all images from the Docker Hub have a docker.io/ prefix before them:

[root@61118d2761ed ~]# docker images
REPOSITORY                   TAG                 IMAGE ID            CREATED             SIZE
docker.io/busybox            latest              d20ae45477cb        19 hours ago        1.13 MB
docker.io/postgres           9-alpine            e9e9c4470522        13 days ago         37.7 MB

I’m used to this syntax for images from custom registries, but not from the Hub. Is this caused by a config option? This doesn’t appear to be new functionality added in a specific API version, because the Fedora 26 box above is using API 1.26, while my laptop running Arch is API 1.30 and it doesn’t have the docker.io/ prefix.

I am seeing the same, and actually images pulled earlier don’t have the prefix and newer pulls of the same image from docker hub do have the prefix. It created considerable confusion (the image seemed not to be reflecting changes in the source) and I still don’t see what the cause is. The repository is private, in case that makes a difference.

Did you find a cause yourself?