It seems, Outlook 365 converts whitespace when pasting.
Screenshot of notepad ++ before pasting into Outlook:
and after pasting into Outlook:
The default font of Outlook 365 (at the time of writing this) is Aptos - which is a non-monospace font. This means, not all symbols (including whitespace) have the same apparent width. Changing to a monospace variant (e.g. Aptos Mono) solves this issue: