Are you using the YouTube app or browsing YouTube in Safari or another browser? I'm not that familiar with anything in YouTube but I see no autocomplete option in the App so I assume that you you are using Safari. If I log on using Safari I have no issues with the "autocomplete" from the drop down choices.
You can try clearing Safari and see if that helps.
Go to Settings>Safari>Clear History, Cookies and Data. Restart the iPad. Restart the iPad by holding down on the sleep button until the red slider appears and then slide to shut off. To power up hold the sleep button until the Apple logo appears and let go of the button.