The problem is that JIRA "resolves" the "Automatic" assignee before it checks for mandatory fields.

So if the Components field has been set mandatory, and the component leads have been nominated, and the user creates an issue without selecting a component, JIRA resolves the "Automatic" assignee as if no components have been selected (e.g. to the Project Lead). Then JIRA validates all mandatory fields, detects that components have not been selected and returns the error page. Unfortunately the assignee already has been resolved, so the Assignee drop down has e.g. project lead selected, rather than "Automatic". The user needs to manually set the assignee to "Automatic" again (as well as select the component), to get the issue assigned to the right component lead.