According to the Ben 10 Wiki, 16-year-old Ben regains access to Feedback during the events of Showdown: Part 2:
Suddenly, Ben, in the form of Feedback, emerges from Malware's head. Malware is shocked, but says that he will destroy Feedback again.
It's unclear to both me and the Wiki how 16-year-old Ben was able to do this:
[Feedback] is one of the four ...