Sian my left JoyCon analog stick seems to be having problems.
Pushing the stick down doesn't respond as fast and letting go the return is slow, sometimes it will drift up a bit also even at rest.
Noticed it when playing Mario Tennis sometimes my player run up for no reason.
mine will tend to move left sometimes. play until pek chey, decide to buy new one. i realise like left joycon seems to be the most common problem when i ask around.