How do I stop my phone from automatically deleting old text messages?

I have an LG q7+ cell phone. I noticed that once I get a large number of text messages from one person, it automatically starts deleting the oldest text messages from that person whenever I recieve a new one. How do I get my phone to stop doing that without deleting messages myself? My LG Q7 + has 64 GB of memory and I have only used 25GB of space. So it's not a lack of space that causing this. My old phone never did this! My previous phone was a ZMAX Z970 which only has 16GB space. So 1/4 the space and it never automatically deleted messages. So why does my new phone automatically delete the oldest messages? How do I fix the problem?