Have you tried visiting calendar.google.com from a browser, logging in to your account and checking whether the calendar events are there. If they are then you just need to sync the new phone with them and it's done, doesn't matter about the old phone. There is a "bin" there, so it might be worth looking for deleted events there if you can't see them. There's also an "import and export" option in the settings there, so if you can find the events you could export everything so that you have an independent backup.
If the events weren't synced from the old phone then their being deleted from the old phone is the end of it. One of these setup apps (each manufacturer tends to have its own) shouldn't delete anything from the old phone, so something clearly went wrong, but that doesn't help now that it's happened. I don't think it's a "synced the wrong way" issue since they don't sync (two-way transfer), they just transfer (one way), and everything else transferred so you didn't do the process the wrong way round. But unfortunately that just leave "a bug in the app" as the explanation, which isn't something that offers any solution.