I just posted a comment about this without looking to see if anyone else had. How foolish of me. But he loses it in "Finn the Human" but was that really in the future? I thought it was more of an indication of alternate universes.

This is the second time this season we've seen Finn with a different arm, the first being the robot arm in the season premier. It seems to be a major signal of alternate universes, and something to look out for in the future.