I don’t know when it stopped working exactly, but when I have a String input inside a Metanode it is no longer giving me the string input field in the metanode configuration window. It used to be there automatically, did this process change?
Since 3.0 you need to use a Wrapped node and not a Metanode anymore.
Right click the group of nodes and select "Encapsulate into Wrapped Metanode".
Wrapped metanode are self-contained. You need to specify which flow variables to let in and out of the wrapped node. Thsi can be specified in the input and output node inside the wrapped node.