Why do partitions get unmounted automatically after some time?

I have an Android phone running Oreo 8.1.0. I have two partitions in sdcard. One partition is ext4 formatted. I mount it using: mount -t ext4 -o rw /dev/block/mmcblk1p2 /data/sdext2 But after some time, the partition gets unmounted automatically and I have to mount it again. Even when it is mounted, the contents are not visible in file explorer apps like MiXplorer and ES Explorer, and are only visible in terminal. So, what is the best way to mount ext4 partition in Android persistently and what is the reason of such behavior?