Do you have auto delete enabled?
Have you tried wiping the cache partition of your device using the steps below to do so?
1. Turn off the device.
2. Press and hold the Volume Up key and the Bixby key, then press and hold the Power key.
3. When the Android logo displays, release all three keys.
4. An ‘Installing system update’ message will show for 30 – 60 seconds before the Android system recovery menu options appear.
5. Press the Volume down key several times to highlight wipe cache partition.
6. Press Power key to select.
7. Press the Volume down key to highlight yes, them and press the Power key to select.
8. When the wipe cache partition is complete, Reboot system now is highlighted.
9. Press the Power key to restart the device.
Have you tried clearing the cache and data for your messaging app?