Problem :

Organisation administrators sometimes need to retrieve the corresponding username for an Atlassian account ID. This is quite cumbersome for admins because userName is a supported filter on SCIM, but there is no userName attribute on Atlassian Accounts. 

Suggestion :

Support Atlassian Account ID as a filter on the SCIM Users endpoint

Workaround :

The only way to retrieve this information from Atlassian is to go through all the users in the directory and do a search for each set of results.