GitHub chooses to do this. It's GitHub's choice to generate Source Code files on demand rather than when the release is made. It's a way of reducing their disk usage at the cost of this kind of potential problem.

The problem is they also presented it as if it was a stable reference. If people knew it was not stable they would have done what the Bazel devs are now talking about doing, which is also uploading the source code at release time, as an artifact (which is how it works on Nexus).