Configuring preferences and options in Outlook.com
I take it back.
For me I suspect it is the E key - actually just lowercase 'e'.
I think the problem stems from the fact that Outlook 365 allows single keys to impact a selected email.
e.g. when you are using the list of emails pane, 'e' archives a selected email; 'j' marks it as junk; 'u' marks it as unread; etc.
This to me is a fundamental flaw in their programming: one should never use single keys for such operations, but rather CTRL+e or OPT+e or CMD+e, etc.
Granted, the browser has already 'stolen' a bunch of hotkeys, but there should be enough left to allow browser-based software to have its own unique hotkey combos.
Anyway, for some reason, I think that in the middle of writing an email, Outlook 365 interprets a key as affecting the list of emails pane (and thus whatever email is selected there), rather than the currently-writing-an-email pane.
Perhaps SHIFT confuses the software into thinking the next key belongs to the list of emails pane?
I thought I had gotten the SHIFT then 'e' to reproducibly archive emails, but I think the issue is more intermittent and (much like taking a car to a garage for a weird noise, only for the mechanic to find nothing out of the ordinary) not readily reproducible.
In any event, clearly it's a bug in the software, affecting plenty of users, so Microsoft need to come up with a better solution than just disabling all hotkeys or altering registries. https://support.microsoft.com/en-us/office/archive-in-outlook-for-windows-25f75777-3cdc-4c77-9783-5929c7b47028