I solved this problem, change the 'DefaultSelectedItems' property to:
ForAll(Filter('Tier Board Action Plans - all text columns', ID = Gallery1.Selected.ID), {Result: ThisRecord.Branch})
btw, the Item property is:
Sort(ForAll(Distinct(Filter('Tier Level Mapping', 'Super Branch' = Dropdown7.Selected.Result),'BU Name'), {Result: ThisRecord.Value}),Result)
However, even though I solved this issue, I don't understand why I have to use ForAll and {Result: ThisRecord.Branch}. Could anyone help me to understand what happened inside?

Thank you!