Hello,
I do partially agree to the fact that this is not a bug, because it works well as it was designed. However if this is the way it was designed, I cannot agree that this is a normal/usable design for an email client.
When composing an email, either it being an answer to a previous email or a new email, you very very often need to check something in another email stored somewhere in Mail.
Designing the compose "window" (I put quotes because it's not really a window) as modal (ie you cannot click on the main Mail window to put it back in focus et navigate through your folders and then come back to the compose window) makes it almost unusable (unles you compose a simple email with no other need). When email is one of your main task, having to esc full screen mode to be able to check another mail is a nogo ...
I guess that, one again, design did prevail over usage (because the drawer-like modal window is of course more fashionalble than the classic window), and this is happening a little bit to much these days with Apple products (don't get me wrong, I am a Apple and Mac user since 1986 .. and that won't change anytime soon).
When I buy a product, I like to think that I can use each and every one of its functionnalities, some I might like , some not, but I am not happy with functionnalities that cannot be used and that I did pay for. For me fullscreen is useless in mail and I did pay for it (it's not that much my 30$ , but all that work that went into Mission Control and fullscreen mode that seems useles ...)
Anyway, I will send my thinking to Apple.
P.