I think it's when you evolve your buddy, the game removes the buddy status to evolve so it resets the distance as if you removed it and replaced it Up 0 Down
It does reset the progress to get candy, yes. But it keeps the total distance you walked. You just lose one candy in a silly way. I've been there... Up 0 Down
Charizard didn’t forget. He just changed his attitude to Ash thinking he wasn’t a good trainer. Up 0 Down