Hi @amirjpr
Try loading some data onto the SD card then use the phone in [https://www.samsung.com/ph/support/mobile-devices/how-to-use-safe-mode-on-samsung-galaxy-device/|safe mode] for a few days and check if the data remains on the card.
The phone may be a bit awkward to use but doing this will help to isolate what the problem may be.
If the data is still there, then a ''downloaded app¹'' may be the cause of the problem. The trick is to find out which one. You will need to uninstall each downloaded app one by one and then test the phone in normal mode between each uninstall to see if it is OK or not. You may have to allow a few days between each uninstall to know for sure. When the phone does work OK i.e. data is not being deleted, then the last downloaded app that was uninstalled was the problem.
¹A ''downloaded app'' is one that you downloaded and installed on the phone and wasn't pre-installed on the phone when you first got it.