I have the same issue on macOS using iCloud sync. I have Obsidian installed on iOS as well, but nearly never used.
Problem started a few weeks ago and at first I thought I did something weird with keybinds or whatever because it’s only content(text) that disappears, not the file itself. I was in most cases able to recover content by pressing Cmd+Z. When irritation reached a medium-sized level I started to troubleshoot a bit and it seems to me that it is iCloud doing something funky.
I’m not quite sure what happens under the hood, but I have a screen video of me reproducing the issue if any mods are interesting in having a look. If so, please let me know where you want me to send it.