Consider a file with creation date D1 and (different) modification date D2. From either a Mac or Windows PC copy such a file over SMB to FreeNAS 11.2-U5.
The modification date D2 is preserved, but the creation date is lost and also set to D2.
I tried the same test copying from Mac to Mac, Mac to Windows, Windows to Mac, and Windows to Windows. In all cases, both dates were preserved. So FreeNAS' behaviour is anomalous.
ZFS itself supports creation (aka birth) dates, so that's not the problem.
FreeBSD though seems to have poor support for creation dates, which I guess is the root cause.
(This is a shame, because I'm about ready to copy 15 years of data from an old Mac file server to a FreeNAS, but looks like I can't do it losslessly.)