Does iOS now delete text messages on iPhone without warning? It appears that my iPhone 4s have silently and completely by itself deleted a number of messages from the beginning of this 2012. In the place of a longer messaging thread, there's now a load more messages button in the affected text conversations, but it doesn't actually load the previous message history. 

I am nowhere near any sort of limits storagewise, so I don't see why the iPhone would perform any housekeeping on its own. Digging into this, it appears that only the conversations using iMessage have been cleared out but not the ones that only transfer via SMS.

Is this a known behavior that is either documented officially by Apple or currently an undocumented feature?
Have you changed any of your iMessage details in that time frame? 

As I understand it, if anything changes in your iMessage settings (for instance, removing a 'receive at' or changing the account you use, your phone will purge all of the messages it no longer thinks belong to it (as, I suppose, you would expect). However, I added an email address to mine fairly recently and all my iMessage history with my mum vanished. 

I would check with the other party and see if they still have the record - if so, then your phone has elected to purge it and probably thinks you don't need it. If not, is there a time limit on iMessage storage on Apple servers? (seems unlikely and that shouldn't cause it to purge from the phone).