Hello @v-xida-msft
Thanks for getting back to me.
I tried your solution however, it didn't like.
I have two SharePoint Lists, one covers our Support Tickets and another being a master list for all locations we deal with.
This PowerApp is a nicer interface for our Support Tickets and uses a lookup column to the locations list so that we can note the locations impacted by any issues.
On the page, I have a gallery with all of our tickets which when a ticket is selected, it populates the data within the Ticket Info section:

The Responsible Depot/Port/EDI Vendor Combo Box has the below within the Items section:
SortByColumns('Logistics Locations',"Title",Ascending)
Within the DefaultSelectedItems I have the below:
Gallery1.Selected.'Responsible Depot/Port/EDI Vendor'
Which half works, it does pull through the value within the list - however, it does not pull through to the Selected Items as you can see below with the value not showing as selected (allocated numbers so they can be identified)

I have also checked using a label for both one that does work (R_AffectedDept1 is static values i.e. ["1","2","3"]) against the one that doesn't and using as example above (R_ResponsibleVendor1 is SharePoint List Lookup Column)

Hope this clears up any confusion, if not please let me know.
Thanks
Bradley