$expand=variables has no effect on https://docs.microsoft.com/en-us/rest/api/azure/devops/release/releases/list?view=azure-devops-rest-6.0#releaseexpands

While all other expands seem to work for the release list, the variables expand does not. One is forced to fetch the details on the release resource itself. This is not really a spec issue, i guess this is rather an implementation bug on the backend. I'am not sure how to report those - is this the right repo?

Start with the Azure DevOps release list endpoint in the linked REST API documentation and verify the reported $expand=variables behavior against the other release expansions. Determine whether the missing expansion is represented in this specification repository or belongs to the backend implementation; done means identifying the correct ownership and a reproducible next step.

Written by the indexing model from the issue text.