When creating a mapping in OpenIDM. If you create a transformation, OpenIDM does not allow you to select an empty source object. OpenIDM will instead default and select a source.
This breaks scripts like the following:
source.FirstName + "," + source.Surname;
In order to work around this you need to go and edit managed.json manually and remove the source value.
Anecdotally it appears to me that OpenIDM also resets the source if it is blank when you edit existing mappings. So you then need to edit managed.json again. (Not 100% sure on this last bit)