When a build has commits from multiple repositories (such as when using a pipeline job) it isn't possible to determine what repository (remote URL) a commit came from using the REST API.
The issue is neither a ChangeLogSet or a ChangeLogSet.Entry has any knowledge of the SCM it came from. Thus when the bean is exported the API does not have that information. If there are multiple repositories in play it becomes impossible to know the repository a commit came from.
I believe the solution would be to add knowledge of the SCM (or even just remote URL) to the ChangeLogSet when the list of changeset(s) is built (WorkflowRun.getChangeSets() for Jenkinsfiles and AbstractBuild.getChangeSet() for Freestyle jobs). This would allow a consumer of the API to know which SCMs a commit came from.
For example, given a Jenkinsfile job with 4 git repos that only had commit changes for 2 of them the API response might look like the json below. Observe that it there is no way to know which remote a changeset came from. See the attached file for more details.