Doraemon stops pretending to be a sitcom and becomes an actual adventure. Nobita and the crew climb Shinugumi Mountain not to learn a lesson about friendship, but to survive it, and the shift in tone works because Doraemon himself changes, suddenly he's not there to fix everything with a gadget, he's there because his friends might die. This is what happens when you take a gag manga's premise seriously for ninety minutes: the emotional weight lands because you've spent years watching Doraemon as a cushion against the world.
Doraemon: Treasure of the Shinugumi Mountain is a 1986 adventure and comedy and fantasy special.