Nope, not a bug. Working as intended.
The original draft (started in another app, synced from server) may contain arbitrary HTML, including something that my rich text editor won't be able to handle it.
So in order to preserve your carefully made styling (Thunderbird, Outlook, web mail), I show the original message separately, so it won't get messed up.
Any new text you add above will be merged with the existing text, as usual.
However, please note that:
- The subject doesn't get changed the way it does when replying / forwarding (no "Re:" or "Fwd:")
- The original text doesn't get indented and doesn't get the replying / forwarding header ("such and such on this date wrote:")
So, the original message text, coming from another mail app, with possibly very nice complex formatting, will be preserved exactly as is. That's the idea here.