group-reply (ctrl-g) only replies To: but not Cc: ?

[HawKing at bitmessage.ch]

I' m not sure if this is supposed to be default behavior, but when I
attempt to group reply to an email, only those addresses previously
listed as To: are included in the response, and not those Cc'ed. Is
this a bug, something I may be suppressing, or something to enable? I
have not been able to find an answer anywhere online. 

Mutt -v
8.1.1 20180712 (Red Hat 8.1.1-5)

Thanks!