Does anyone know if moving selected choices to the top of the multiple-choice dropdown is a good design pattern? This would prevent users to scroll through a lot of content to preview active fields.
Below quick example - before and after:
It depends on the context, personally, I don't think it's a good alternative. Multiple-choice options with text are usually arranged in an order to facilitate searching, usually alphabetically. I easily find Spanish avoiding all the content by going to the "S".
If it's a filter, there are alternatives to display the selected options in a long multiple-choice by nesting them at the top:
Example via myfonts
I would not recommend dynamically moving the position of selected items in a dropdown menu. When the user selects the item, one of two things would have to happen:
Based on your comment to Danielillo that there might be 30-40 potential criteria in your filter, you might want to explore showing your filter or facet in a panel vs. having it be contained in a dropdown menu. You could add headings to group criteria if needed.