According to https://confluence.atlassian.com/jirasoftwarecloud/advanced-searching-fields-reference-764478339.html#Advancedsearching-fieldsreference-AssigneeAssignee its possible to use Full Names or Display Names in JQL for at least the system user fields.
You can search by the user’s full name, ID, or email address.
However, this is no longer working in any of the Cloud instances I have access to. Has Atlassian changed something?
EDIT: I’ve raised a DEVHELP ticket about this