Dropdownlist filtering does not work properly when datasource is loaded async Completed

Strange behaviours in several code solutions (scenarios), but none of them as aspected.

Expected behaviour: Field is displayed with a default value  3 (comes from database) with text: Initial Value. On DropDown the list of possible values are collected with an API call from the database. (There are several dropdowns on the screen, so collectiing all the data in front is not an option, maybe the user wants to change 1 field).

Scenario 0: List is populated on BeforeOpen and Initial value is not implemented. List is not shown on first open, when filtering the filtered items appears, but when filter is cleared no records are shown. When the list is reopened the filtering works as aspected.

Scenario 1: Is my initial solution, but the list is not populated with the data from the API.

Scenario 2: After some debugging I discovered that the list is populated when the dropdown was closed en reopened. The trick is to cancel the beforeOpen and reopen the dropdown when the data was populated. But this works only when the initial value was not populated in front.

Scenario 3: the initial value is displayed correctly, but the list is not displayed (only the selected (initial value) when filtering the filtered values are displayed in the background when a value is selected and the dropdown is reopened only the initial value and selected value are in the list.

Not one of the scenarios is the solution of expected behavior.

The scenarios are implemented in 

Regards Arnaud