How do I fix missing messages in group chats on my iPhone?

I am in a group chat with a few friends, and some of them have iPhones as well. When any of them send a message in the text chat, I get the notification on my lock screen and can preview the message. When I open the text group in Messages, none of the messages are present, and the group chat has reverted to a specific message from several months ago.

Every now and then, the texts will show up, but it always eventually reverts to that same message from a few months ago, as if nothing had happened in the group chat since then.

[Re-Titled by Moderator]

Original Title: Messages after certain date won't show