But, wait: even if the above is true, then only one of the regions would have an application that knows about the device? Or did you create multiple applications (for different regions) that know about the very same OTAA device, and use the same DevEUI, AppEUI and AppKey? Only when all match, multiple handlers could accept the OTAA Join?
(When you cannot change the keys in the device, then be sure to read How can I reuse hardcoded AppEUI and AppKey after I deleted an application? - #4 by htdvisser before deleting anything.)