Text messages disappearing on my iphone ... freaking out

All of a sudden, text message histories I've had for years with my friends are getting wiped out randomly, and I don't know why.

In some cases I'll have the last message with them, or in some cases they're just gone completely.
I have the option checked in my setting to keep text messages forever and I have 32GB of free space on my iphone so I'm really confused as to why this is happening.

Any ideas on how to get this to stop. I've kept these text messages for years for a reason, and having them randomly disappear like this is really making me upset.