Apple says that an iMessage bug fix is coming soon
Apple has acknowledged that iMessage is buggy for users when they switch to Android or another OS. The bug makes it difficult for Apple staff to keep track of which phone numbers are currently associated with an iPhone, which is essential for the proper running of iMessage. Former iPhone users may find that his or her messages are being accidentally treated as iMessages and never get delivered. Apple said that a solution will be made in a “future software update.” If you want to switch from an iPhone to another device soon, the best thing to do is manually disable iMessage and return to regular SMS.