I see you were fortunate enough to not use notepad aprox 5 months ago, when they were running the rich formatting preview. It was on by default, and would drop around 5% of the characters you type. Literally failing at the only thing it's supposed to do. I repro'd this on 2 out of 2 machines.

Maybe they fixed it, maybe they haven't. I both turned off formatting and am using vscode for notes now.