There is a long write-up here: Diablo 3 Forums
This came up again because of a recent Challenge Rift that relied on frequent use of the target lock, but on the Switch the target lock is on a button that is inconvenient to use in conjunction with the movement joystick. This placement differs from D3 on PS4 and Xbox One and it also differs from the assignment used for the target lock feature on other Switch titles.
Can we fix this, or get an optional alternative controller mapping for those of us used to PS4/Xbox/other Switch games?