Disappearing iMessage conversations on iPhone

So when i talk to this person who we are gonna call Kay mind you we talk everyday ever since im pretty sure March or April and in total we have 4,615 messages and I’ve noticed when i tried to scroll up to see it only shows 69 of those messages which is from Monday and At some point it shows all our messages and the next minute it shows the 69 messages that are there and this problem occurred yesterday evening im pretty sure and I haven’t deleted any type of messages between us at all so can someone explain if its a glitch or if my phone has been hacked? Please help me out so I can try and get all these messages back thank you.

[Re-Titled by Moderator]

Original Title: Ive noticed on imessage there is this one person i talk to everyday and i noticed when i try and scroll up through our messages it shows very few of them even though we talk everyday and we have over 4,000 messages and the next min it goes back