topsort | Topological sort implementation in JavaScript | Frontend Framework library

When I try to implement a topological sort in C++ with an unordered_map> which represents the graph, I encounter an unexplainable error (from my part). Specifically, this happens only when the 'current node' that is being visited does not exist as a key in the unordered_map (i.e, it has no outgoing edges). Instead of returning the 'correct' order, it terminates the function call topSort entirely and returns only a small subset of the order.

The code returns: ML, AML, DL

Instead, a possible correct solution could be: LA, MT, MA, PT, ML, AML, DL

Can anyone explain why this happens?

The following is a small code snippet where the problem occurs: