[Samba] "force group" rule not obeyed

[Thierry ITTY]

I have 2 shares with "force group" rule

when I copy/move a file from one share to the other (using mouse drag'n
drop in windows explorer for example), the new/moved file has the group
specified in its share "force group" rule

but if i open a file on one share using some file/image editor (tests made
with adobe's photoshop) and do a "save as" with a new name on the other
share, the new file keeps its old group (as enforced by the other share's
"force group" rule)

any clue ?
any idea how i can solve that ?

tia