Same issue.  Galaxy S8.  Started after latest Android update.  Messages from iPhone and Android platforms in group chat get stuck in the "downloading" status.  I have taken all recommended actions in this thread.  Nothing works.  I discovered this problem in the worst possible way: when trying to connect with friends during a festival weekend.  I wouldn't get any messages and then run into one of the people in the thread who has been trying to find me for a meet up -- look at the phone and a bunch of messages "downloading" and no notification.   I just cannot tolerate this problem - I need to know to fix it right now.