The "Show One Journal Field" toggle is not working on forms. The button appears to be completely unresponsive.
Any release or environment that has UI16 enabled.
If there is anything that is making one of the Journal Fields mandatory (ie. UI Policy, Client Script, Dictionary Override, etc.) the toggle option will not work. This is working as development designed since they do not want users to have the ability to hide a mandatory field. If you make a journal field mandatory, the activity stream will show all of them.
You can either remove the mandatory requirement on your Journal field, or keep your journal fields expanded so that both are displayed at all times. There is no option currently to collapse journal fields into one section while one of the fields is mandatory.