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Finally I understand from AMD website, the reason is I turned on the DisplayPort1.2 with one monitor.
In order to assist and inform end users about DisplayPort 1.2 MST bandwidth allocations, AMD has provided a new user interface. When an MST-capable device is connected to a DisplayPort output, a “DisplayPort Diagnostics” page becomes visible in the advanced view of AMD CatalystTM Control Center. This page provides information on the total available bandwidth for the connection, the amount of bandwidth currently being used on the connection, and the amount of bandwidth being used by each individual display.
Glad you sorted it.