Results of the Dropdown will be populated along with the section rendering hence the section parameter will be available for the datapage. This is applicable to "At screen load" load behavior only. Kindly check the behavior with other load modes, you should see the same behavior as autocomplete.
In case of autocomplete results will be populated when we start typing in the input field hence section parameter won't be available for datapage.
As a workaround set a property with the parameter value during section rendering and configure that property reference as the parameter to datapage.