Deleted ~/Library/Application Support/TorBrowser-Data, deleted TorBrowser.app. Reinstalled 7.0a3 from TorBrowser-7.0a3-osx64_en-US.dmg. Search still defaults to YouTube for me.
I just rechecked a freshly installed 6.5.2 from TorBrowser-6.5.2-osx64_en-US.dmg (after deleting TorBrowser-Data) and it defaults to DuckDuckGo as expected.
Thanks for testing. This does not happen on Windows either which makes me wondering whether that is a macOS-only issue as #21879 (moved) with the same underlying root cause.
Trac: Summary: when I use the search box in the upper right it defaults to using youtube. to When I use the search box in the upper right it defaults to using youtube (on macOS)