Currently /rest/api/2/workflowscheme/project only works for projects that don't have issues.
We would like to be able to use this endpoint for project that have issues by mapping statuses for migration as we do through the UI or as other endpoints do it (https://developer.atlassian.com/cloud/jira/platform/rest/v3/api-group-workflows/#api-rest-api-3-workflows-update-post

https://developer.atlassian.com/cloud/jira/platform/rest/v3/api-group-workflow-schemes/#api-rest-api-3-workflowscheme-update-post)

Documentation for /rest/api/2/workflowscheme/project
https://developer.atlassian.com/cloud/jira/platform/rest/v2/api-group-workflow-scheme-project-associations/#api-rest-api-3-workflowscheme-project-put