You have identified what many critics call the biggest disappointment in this major American tour de force. Most critics agree that this last episode of the novel featuring Huck's reuniting with Tom Sawyer and the Phelps' farm is a real bathos or anti-climax given the central chapters that chart Tom and Jim's adventures together by themselves and the characters they meet. They argue this because the central chapters demonstrate the development of Huck Finn as an interesting and valid character whose attitudes and assumptions towards social institutions of the time reflect the criticisms and doubts that Twain himself had. Key to this development is the relationship between Huck and Jim as a slave. Huck seems to constantly struggle to choose between what society demands of him and what he feels instinctively is right. This is seen for example when he determines to turn Jim in as a runaway slave because that is what society says, but in the end he chooses to follow his own heart and does not.
However, the Phelps' farm episode acts as a reintroduction of the Tom Sawyer storyline, which, albeit amusing, lacks the depth and profundity of the central chapters. Huck returns to being a mere sidekick to Tom Sawyer, even though he has shown himself to be more than capable of making decisions for himself when he was with Jim, and now seems a redundant character, having forgotten all the character development that the central chapters pointed to. It appears that Huck as a character has reverted to the Huck we were first introduced to at the outset of the novel - a character who is more than willing to be led by the more charismatic Tom Sawyer, in spite of the ridiculous schemes and plans he hatches, and a character who is not willing to take the initiative himself or share any of the insights he has learned on his journey.
There is a definite shift therefore from the central chapters to the amusing and ludicrous "evasion" scene where Tom and Huck embark on a series of far-fetched plans to free Jim, only to find at the end that Jim has been freed all along.