If the user assigned in the Assignee field is not available on the destination
If assignee user is not available on the destination and default assignee (project lead) does not have assignable user permission
If the assignee is not available in the destination and the default assignee is set to unassigned, but the "Allow unassigned issues" option is turned off (configurable in general settings),
If the assignee is not available in the destination, the default assignee (project lead) lacks assignable user permissions, and "Allow unassigned issues" is turned off (configurable in general settings).