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 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