It seems this is a situation where it is not possible to map the target and source keys directly. I would suggest you to refer relevant User and Technical documentation ( About Object Connection Transformation & Technical documentation on OCT.)

For your convenience I will paste most relevant instruction in the above documentation.  

“ For some scenarios it is not possible to map the target and source columns due to the fact that the source columns are not included in the target database view. You then have two options which both require some programming work. You can either extend the target view with the necessary columns or create a stored function that makes the actual key transformation. The name of the function should in this case be registered as the transformation method. Note that it should also hold the package name, e.g. My_Package_API.My_Transformation. It needs to be defined as a function that returns a string value. It also requires two values in parameters (strings); the Target Key Reference and the Service Name (in that specific order).

A custom event + event action can also do this, but the end result is slightly different, and static, unlike Object Connection Transformation.

Isn’t the real problem here that Work List Line document connections are not copied from Work Task Template to the Work Order Work Task tab?

Documents connected to the Work Task Template are copied, so I would expect the same for the Work List lines

@AddEysteE 

Perhaps, yes. People expect all sort of things, it just takes time implementing all of it  

Joking aside, if we should use an Object Connection Transformation rule or make a proper copy of the document connection depends on the use case. In some cases we want to use a rule, since it's dynamic, and in other cases it can be critical that the "target" object gets it's own document connection. I don't know the area well enough to say what is best here, if there is an objective answer to that.

Thanks for the comment!