specified sha does not exist

The past few days, certain CI builds have been failing with the status_message showing something along the lines of "specified sha 'ff1176468d368232b684f75e82563369208bc371' does not exist".

https://github.com/ClangBuiltLinux/continuous-integration2/actions/runs/1069378557 (example)

https://github.com/ClangBuiltLinux/continuous-integration2/actions/runs/1065735887 (example)

The first one does not make any sense, as the SHA is the Linux 5.14-rc3 commit, which is obviously in the repository. Did something change on the backend?