This JIRA is a follow up of AF-2244 in order to change 'X' close icon behaviour:

According to Amy:

Any notification that doesn't include a link, can disappear and should not have the "X" close icon - so the user does not have to take action to dismiss.

Any notification that DOES include a link (in this case, to View Execution Details) should only be dismissable by the user -either by clicking the link or clicking the "X" close icon.