Summary

According to this documentation, you should be able to set the assignee via accountId, but doing so does not correctly set it.

Steps to replicate

1. Create an issue and make sure that it's unassigned
2. GET /myself to get the accountId
3. cUrl request:

   curl -X PUT \
     https://<instance_name>.atlassian.net/rest/api/2/issue/<issue_key>/assignee \
     -H 'Authorization: Basic <basic>' \
     -H 'Cache-Control: no-cache' \
     -H 'Content-Type: application/json' \
     -d '{ "accountId": "<accountId>" }'
   

Expected behavior:
Should be able to update this via accountId