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?