What is the general goal of the feature?
Allow the user to find select multiple options in a large list easily.
What can you contribute to making this feature a reality?
I'm not sure why this was not implemented at the same time as search in select one. One issue compared to select1 would be that if you are filtering the choices then the shown list may not include all the selections you have made. This would not be desirable. One solution to this could be to show 2 lists on the page.
List 1. All choices filtered by the search criteria with both checked and unchecked choices shown. The user can check and uncheck choices in this list.
List 2. Only the selected choices. Users can un check from this list after which this choice will be removed.
Possibly the above setup would also be a good UX for select multiples that don't have a search filter. Ie if you have 30 choices and you have selected 3 it might be nice to see those 3 selected choices in one place without having to scroll up and down and try to get your eye to follow across from the checkbox to the text to verify that you have selected what you want.