Can you provide more information, may be with the code snippet that you have?
I am guessing this is related to: https://github.com/jenkinsci/jira-steps-plugin/pull/103
jiraAssignIssue idOrKey: issueId, userName: 'user', site: 'sitename
It is related to GDPR changes, now we can not use username in Jira api for assigning issue to particular user. In jira-steps-plugin only username can be used for assignment.