[Samba] weird permission problem

[Thierry ITTY]

I have a samba 2.2.7a on a linux server 

I have 2 shares, say shrA and shrB

in smb.conf I force group grpA on share shrA, and group grpB on share shrB

[shrA]
	path = /shares/dirA
	force group = grpA
...
idem for B

user userU is connected to both shares from a windows station
he has a file fileF on his local disk C:

he puts file fileF from his local disk on to shrA (via a mouse drag'n drop
for example)

on the server ls -l /shares/dirA shows :
-rwxrwx--- userU grpA ... fileF 

then he puts file fileF from his local disk on to shrB (the same way)

on the server ls -l /shares/dirB shows :
-rwxrwx--- userU grpB ... fileF

everything's fine till here

no he erases shrB/fileF and copies fileF from shrA on to shrB

again on the server ls -l /shares/dirB shows :
-rwxrwx--- userU grpB ... fileF

everything's fine till here again

now if instead of moving/copying the file from one place to another he uses
some kind of software's function "save as" (actually he works with adobe's
photoshop) then the force group doesn't work anymore !!!

he double-clicks on shrA/fileF, photoshop starts
he changes whatever he wants in the image
he goes to the menu bar and chooses "save as" and saves to shrB as fileF2
the problem is that now we get on the server ls -l /shares/dirB shows :
-rwxrwx--- userU grpB ... fileF
-rwxrwx--- userU grpA ... fileF2

despite the "force group = grpB" for share B

and still more amazing, while photoshop was saving it was using a temporary
file on shrB with the right group !!! only fileF2 was keeping its original
group !!!

any clue appreciated !

tia