I have also found this a problem (and have reported it via the web editor feedback button). Here is my experience:
It happens when I use the web client. (I've not experienced it on the Windows client.)
It happens in Firefox and Chrome, both on Windows 10.
It happens on different Windows 10 machines.
I have only noticed it since the Beta Editor was introduced. But...
It happens on both the Beta Editor and the Classic Editor.
Here is what happens:
I am typing in the middle of a text note (in Beta or Classic editor of the web client).
At some random point the following things happen in quick succession:
The last few keystrokes are suddenly undone. If I was typing this means the last few characters disappear from the note. If I just pasted a bit of text that might disappear.
The cursor jumps to the start of the note.
The screen does not move to the top of the note until I type the next characters.
Note that those things happen in quick succession. So - if I am typing without pausing - it looks like the cursor has just jumped to the top of the screen in the middle of typing. But on closer inspection - when I scroll back down to where I was typing originally - I also find the few characters before what's at the top of the screen have disappeared. For example, if I type "Once upon a time" at the bottom of the document and 1, 2, and 3 happen just after I type "upon a" then I will see " time" at the top of the screen, scroll back to the bottom, and see just "Once up".
A slightly different effect happens if I am typing and the bug occurs a moment after I pause to think. In this case, steps 1 and 2 have happened, but 3 has not yet happened because I've not typed anything more. In this case the effect is that I will see my typed text, I will see the last few keystrokes mysteriously undone before my eyes(!), and the blinking cursor after that text will have disappeared. Scrolling to the top of the note I will see the cursor blinking away there.
Yes, this is very annoying, and yes, it does make it near-impossible to work in the web editor.