On our on-premise TFS 2017 update 2 I want to add an extra user to the Agent Pool role as an administrator. For some reason I can not find all users in the domain. 
In the [image](https://i.stack.imgur.com/8SmPl.png) you can see that the user TFSBuild was added previously, but now when when searching or it it can not be found. I know for certain that this user still exists because in other place (e.g. add it to a a project group) I can add it. Also I'm able to add other users so the permissions to add users to the administrator role are OK.

I noted that I can add the user to the 'Project Collection Build Administrator group' but I still cannot add it to the Agent Pools or it doesn't show up in the 'filter users and groups' of the collection