If you create python node with only one input in a Custom Node workspace, there exists a graphic glitch where the height of the node window is cropped. This means that you cannot actually double-click on the body of the node to open the Python editor window. You need to add a second IN in order to create enough background whitespace to click on. This does not happen if the python node is created in the regular workspace. In that space, the window has additional space below the IN[0] input variable to allow double-clicking.
Best to report this to the GitHub page as an issue.
It is possible to edit the Python code by right clicking on the node and choosing edit....
Mine does this as well. There is a little strip below the plus and minus where you can click. Its not too hard to click on it. I have not noticed any problem with it.