Using classic Outlook for Windows in business environments
I had single line in NEW Outlook and then I didn't, and got frustrated for a while and found this discussion. Unexpectedly after trying so many things and breaking for a day, I discovered something that fixed it.
If the reading pane is hidden (view>layout>reading pane), the mail shows in single line for me.
If the reading pane is visible on right, in windows the width of the middle column can be grabbed and stretched right a bit, enough to change the layout from multi-line to single line layout.