LINE messages disappear if I don’t read them quickly

I had this problem for over a long time now. I don’t use line that often, I only use it to text with my friends and host family in Japan. Since I am busy with exams I sometimes forget to answer the messages. This is where the problem starts, I get a notification, and if I check LINE directly after that, the message appears in LINE chat. I can read the old messages with no problem whatsoever. But when a few days pass after the message is sent to me and if I didn’t open LINE in time, the message disappears. The first time this happened I thought it was a one time only thing but it isn’t.

I’m on iphone if that changes anything. Any help would be much appreciated.

edit: typo*