Odd carrier/unlock situation

So, I bought an iPhone 7 plus on eBay. Yes, I'm aware, I'm an idiot. Should've checked the IMEI first. The seller (who won't answer my messages, so no luck there) claimed that the phone was an ATT network phone, and it's been working fine on ATT's prepaid setup for almost a year, no issues. However, I was looking to put the phone on a new, contract carrier, so I did an IMEI check, and lo and behold, my phone is supposedly locked to KDDI out of Japan! Struck me as weird. So, I'm curious to see if this has happened to anyone else, and what the best course of action would be. Again, the phone has been working fine for almost a year, even while supposedly locked to a foreign carrier. Is it actually unlocked, and the IMEI services I've used are wrong, or is KDDI more "open" about these things, or is my phone magic?