Most of my message conversations have disappeared.
When I go into messages, I can only see a few conversations with apparently random people - some of who I haven't texted for ages.
If new messages show up on the lock screen, then when I go into Messages they aren't there. However if I start up a new message to whoever has just sent me the missing message, then my whole history plus the new message they've just sent suddenly reappears!
If I don't spot or recognise who has sent me the new message on the lock screen, then I can't start a conversation with them to find that message.