Configuring preferences and options in Outlook.com
Turning off hotkeys did not fix it for me. However, turning off "Show suggested replies" and "Suggest words or phrases as I type" did fix the problem.
That is odd. It makes sense that something might help, but I’m pretty sure that in the beginning of this issue, there wasn’t any suggested text. Also, most of the times that I had messages disappear, I didn’t have suggested text. They just got archived (with or without an undo feature). It is as this thread suggests: it seems to be that the keys that cause it are not always the same. I’m certain that they keep changing the programming and probably the support team doesn’t know the answer or somebody isn’t saying something.