This looks to be fixed :) Server running Debian Stretch with Samba 2:4.5.16+dfsg-1 and client running Debian Stretch with cifs-utils 2:6.7-1:
[N] ian@draal~> mount /mnt/nas [I] ian@draal~> cd /mnt/nas/ [I] ian@draal/m/nas> echo foo > bar [I] ian@draal/m/nas> cat bar | hexdump -Cv 00000000 66 6f 6f 0a |foo.| 00000004 Cheers, -Ian