I noticed the same problem, with "official" videos not showing up. And it extended beyond the tube app. The browser forces you to use the m.youtube.com, which also will not show the official videos if you search (this is mirrored if you go to m.youtube.com on your desktop browser). When you override this by scrolling to the bottom of the page and clicking "Desktop" in the lower left, you can get to the standard YouTube site. Searching here will show the missing videos, but of course they don't play in-browser, nor is there the download video option you normally get on the mobile site.

The Tube app must only draw on the mobile youtube site, and it looks like the mobile site is not "complete" or lacks certain videos based on criteria. Maybe it's if their HD, or the studio that uploaded them blocks this action somehow. No idea though.