Issues around selecting users in the UI, I have posted about the issue and solution here
https://world.episerver.com/blogs/scott-reed/dates/2018/11/working-around-iqueryablenotificationusers-when-using-external-claims-based-cms-users/
I also encountered the same problem with EPiServer.CMS 11.12.0
Issues around selecting users in the UI, I have posted about the issue and solution here
https://world.episerver.com/blogs/scott-reed/dates/2018/11/working-around-iqueryablenotificationusers-when-using-external-claims-based-cms-users/