Some time ago, Atlassian made changes that requires users to share their email addresses on an individual user profile setting. When that option is not checked, Aha! is not able to view their email address and if assigned user is mapped, is not able to find the user based on their email address. As a result, the assignee blanks out in Aha! Roadmaps.
There appears to be a solution that runs in some cases but not all, that matches Aha! users based on their name when the email address option is not enabled (perhaps this is when it isn't enabled at a server level in Jira rather than individual). Extend that to always run when email address isn't found as a backup measure.