I'm also having this problem. There are hundreds of messages from within group chats that I don't recieve. It leaves a notification in my chat with each individial(that's in the group chat) that a text wasn't downloaded. How do I solve this problem? In some cases If I don't see that notification for a couple days it will then say the messade has expired and can't be downloaded. Unacceptable. 

I'm on Google Pixel 1, Android 9.