When providing his own Groot information, Kevin Feige compared Baby Groot to how Spock was in Star Trek III: The Search for Spock, i.e. when the Vulcan's katra was reunited with his body after it was resurrected by the Genesis matrix. Following this event, it took a while for Spock to return to "normal" (as seen in Star Trek IV: The Voyage Home), so maybe Groot is going through a similar transition. Gunn said earlier in the Q&A that Guardians of the Galaxy Vol. 2 tales place only a few months after the first movie, so when fans reunite with the intergalactic heroes, Groot possibly hasn't recovered his memories yet. However, as the story progresses, he may finally regain them either on his own or through some kind of event. On the other hand, if this is indeed a brand new Groot, then it may be impossible to get those memories back.