Items cannot be unselected by clicking on them in the dropdown of a multi-select with an AJAX data source.

Version
4.1.0

Steps to reproduce

1. Create a multi-select with an AJAX data source. A minimal example can be found here, based wholly on the official docs
2. Type in a searchterm to see a list
3. Select anything
4. Type in the same searchterm as before
5. In the dropdown, click on the item you already have selected
6. The item remains selected (despite an select2:unselect event triggering)

Expected Behaviour
The item should get unselected.