My fstab is correct. Now I know, what was the problem.
I tried to extract one tar.xz with SuperTuxKart via tar xf tool, then via filebox. The app freezed, and I needed to force to close it.
Then, I discovered many damaged files in MyDocs. I wasn't able to delete them even via PC.
So I formatted (while sharing eMMC via BackupMenu to PC) MyDocs to fat32, now it works perfectly, and is correctly mouted at boot