iMessage closes chat every time I exit message app without clearing the app

Hello alyxc96,

Since you're currently experiencing issues with a default app, Messages, we recommend ensure your device is up-to-date and restarting if you haven't already. You can find steps for both processes below:

Restart your iPhone - Apple Support

"How to restart your iPhone X, 11, 12, 13, or later

1. Press and hold either volume button and the side button until the power-off slider appears.
2. Drag the slider, then wait 30 seconds for your device to turn off. If your device is frozen or unresponsive, force restart your device.
3. To turn your device back on, press and hold the side button (on the right side of your iPhone) until you see the Apple logo."

Update your iPhone or iPad - Apple Support

"Update your iPhone or iPad wirelessly

1. Back up your device using iCloud or your computer.
2. Plug your device into power and connect to the internet with Wi-Fi.
3. Go to Settings > General, then tap Software Update.
4. If you see more than one software update option available, choose the one that you want to install.
5. Tap Install Now. If you see Download and Install instead, tap it to download the update, enter your passcode, then tap Install Now. If you don't know your passcode, learn what to do."

These are the best steps as Messages cannot be individually uninstalled and reinstalled to your device.

Cheers.