This is happening to me too. And worse. I don't just get the archiving effect when typing a new message. I also get new messages created. I'm typing happily away in the body of a message and suddenly I have a fresh one in front of me and I'm typing in the address field! I kill that new message, bring up the original again, and then a few seconds later I get another new message again. This is very annoying. It doesn't happen all day, just in bursts.
I think this is associated with fast typing when there may be some latency (delay) in the network link. I'm a fast typist. I might be hitting two keys at once, but I'm not sure this is the case. Maybe Outlook thinks I am hitting two because I type fast. When the problem is happening, I can avoid it by typing slowly, but I think the occasional key gets missed even then. That's why I think network latency (or possibly lost packets or overloading of the server) is making this happen. I have the general feeling that the Outlook web server is getting an incorrect impression of what keys I have hit at this time and/or whether I have hit them together.
I have the impression that the Outlook website reads each key I type as I type it. I don't have this kind of problem with other applications on the computer, so I don't think there is a problem with my keyboard. (It's a really good keyboard, actually, a proper old IBM one.) I have also seen the problem in my office, where I have a different keyboard.
I'm using https://outlook.office.com/mail/
I have Linux and Firefox.
I will try finding out how to turn off keyboard shortcuts, as I don't think I use them. I'm a bit confused by people saying "E" could be a keyboard shortcut. This could not possibly be intended to be a keyboard shortcut in the context of typing the body of a new message.
jen.