To reproduce:
# Open https://en.wikipedia.org/w/index.php?title=Template:Countries_of_Africa&action=edit
# Go to the "Preview page with this template" input box. Type "South Africa". Press Enter. The page "South Africa" is loaded below.
# Close the tab.
# Open https://en.wikipedia.org/w/index.php?title=Template:Countries_of_Africa&action=edit again.
# Go to the "Preview page with this template" input box. Type "South". "South Africa" should appear as autocomplete. Press arrow down till you get to the "South Africa" item". Press Enter.
Observed: The page "South" is loaded below.
Expected: The page "South Africa" is supposed to be loaded.
Either the correct select page from autocomplete should be loaded, or this input box should support MediaWiki's own autocompletion, like the search box.