With the latest snapshot, the sidebar display slot no longer changes, regardless of what objective you have assigned or are assigning to it.
This is somewhat a duplication of
MC-31777 but since it has been marked as Fixed/Resolved and is in another snapshot, I have chosen to make a new issue.
Use following command to make an objective:
Now, add it to the sidebar with following command:
If you already have an objective on the sidebar (made in earlier versions) it will still update that objectives score, but you wont be able to change it to a new objective.