☒ That was my findings as well. It is rather annoying. I wasn't able to find a suitable solution except to just display what is selected.

If you don't change the value of the select then it will not change the persisted value. But since we are not pre-populating the list, it will allow you to select from the ajax list. I agree, it is rather annoying. I would see this as a defect in slim-select as the same behavior doesn't happen with their multiple select.