The strange thing here is hungary wanting to conquer austria. Now there is the austrian province of 'Burgenland' which was part of the kingdom of hungery until the end of the habsburg monachy. They might want that tiny part back. But that didn't seem to be an option. So why? Hungary was liked it's autonomy from austria a lot. There is a reason why you know the state as austro-hungerian empire. Even the monachists who wanted a habsburgian king wpuldn't want a personalunion with austria at this point.
The allies as in france and britain where far away why should they intervine any time soon. But all of hungerys other neibours had a lot more formerly hungarian terretory than austria and had resently been fighting hungery over it (1919). they would actually be threatend by an attack on austria. So why don't czechoslovakia or rumania intervene?
True that is weird. However they couldn't have known hungery would annex all of austria. Because why should they? Okay the proclaimed war goal to conquer eastern austria (conviniently but unprecisely named lower austria) was already quite ambitious. The plan of the nazis was destabilizing austria to justify an annexation. So the enemy of austrias goverment would have been there ally... No, it's still weird over all. But we can't really expect scriped what if scenarios for whenever the player does something compleatly implausible. And I'm not convinced this is a fundamental mechanical issue.
They sent troops to help against CZ. I proposed already that country can join a war with no penalty if another country where it has core is attacked and this country is not democracy. It should solve the problem.