Well for that I made this little event:
I remove the cores too, so that Hungary does not ask for it back. As well, it will not fire if Germany is human, so Hungary can hang on to its turf, since I assume human Germany is not going to rotate the armies around the Balkans. The only thing I can think of is that somehow the units ordered to go there, in the previous AI (Attack Yugo AI) keep the same target orders if there is no connection to their base.
You would want to change to ID for the event as that is specific to EIR. Once the corridor is opened the armies will then start behaving normally, and head for Russia ASAP. Germany will then release Croatia, but Hungary will not ask for it back because it is not a core.