I can't pin down what is causing this, but I am frequently seeing Evernote losing all formatting of my notes, mainly my most active ones like my todo or shopping list. My formatting is not that involved, mainly newlines and sometimes a single layer bulleted list. When this happens, the note is completely unusable on mobile, or at least very tedious. I have to go back to my PC and reformat the whole note which is the only viable way.
I have mainly seen this when using the mobile app, so it usually strikes me when I need Evernote the most. If I can't trust my data to Evernote then I need to find another product that can do such a simple thing like not corrupting my notes.
Here is an example of what happens:
Sample original note:
Take out garbage
Note after corruption:
--Todo List Take out garbage Buy groceries Cook dinner
I am using Evernote on Windows 10 and IOS (latest). I usually have the Evernote app open on my PC even when I'm away, but even if I closed it, it would still be syncing in the system tray. I only have it on one computer, my iPhone, and iPad, and rarely open anything on my iPad. I use the web once in a while, but not when I see this issue.
EDIT: Just ran into this again and found something interesting. Sometimes the note shows as corrupted on IOS, but on the Desktop it does not. If I edit on the desktop and resync to IOS, the note gets fixed on IOS.