Boostrap interfering in positionning a combo box

I have got a traditional situation where the contents of a dropdown is updating depending on the value of another combo box. It’s working almost as expected. The values are displayed correctly, my only issue is that I can’t set the selected item as the 1st item of my list.

It appears that bootstrap is interfering with it. And I don’t know how to avoid this.

See print screen below, if I remove in the CSS part of the dropdown the value called “selectpicker” the dropdown appears fine as far as the selected value is concerned, keeping that value the selected value appear as the last one, which is not what I want. How can I keep that design but keeping the selected item I want ?