The “XXXX” might give you a clue: does that refer to your own application, or to something unknown? Could the devices have been used by others?
Why is seeing unique AppEUIs suspect? Are those 4 different, or not? (“Seems” is just a bit too vague for troubleshooting, though I assume you mean those 4 are different and, of course, you don’t know about any other devices.) Do they look like true AppEUIs (which might look random, though they are not, but certainly should be a bit complex)? Getting any clue from MAC Address Vendor Lookup: MAC/OUI/IAB/IEEE Vendor Manufacturer Search? (Only the AppKey is secret.)
What does that mean exactly? How did you register the devices? Did you select the proper AppEUI in the device settings (after you first added that to the application)? And also set both DevEUI and AppKey? What do you see in the gateway’s Traffic page, the device’s Data page, and the device’s Status?