Java custom functions cannot be used within a presentation parameter or in an expression editor when defining human tasks. This is not the same as the way in which you use other Java custom functions. If you wanted to use one, you would need to declare the function as being global in the Server Console; you could not refer to them in a contribution. While this works, it can be a problem as users would need to deploy and manage them independently from the package deployment process. Restated, unlike other functions, human task custom functions cannot be just included in a contribution so that a human task process can access it.
While using custom functions within a contribution appears to be a simple way of using them, this limitation prevents them from being easily used within a human task context.
This limitation exists because at runtime, the server expects the task state process to have references for custom function contributions, and this is not possible.