International area codes suddenly appearing in front of my contacts in "Messages" app

The other day I noticed that the international area prefix "+54 9" had started appearing in front of about half of my contacts in Android's native Messenger app. I can still send and receive messages from those contacts (verified with my wife's phone). But it's still annoying, and I worry that I'm getting charged extra by my phone company. When I go into my contacts and try to edit the numbers, the prefix is no longer there, but it shows up again the next time I add them to a texting thread. Has anybody ever experienced this?