'su lyx' changes file access privileges

[paolo m.]

Sometimes, while  in user1 session, I find convenient launching lyx as user2 
(same group as user1)  to open a lyx file, through 'su'.
As user2 changes and save that file, access privileges are changed as well. 
That does not happen when a text file is opened by kwrite, so it should be 
something specific to lyx.
 
Before 'su' switch:
text file     x            -rw-rw-r-- 
lyx file     xx.lyx        -rw-rw-r-- 

Switching and modifying file x (by  kwrite)  and xx.lyx (by lyx):
              x          -rw-rw-r-- 
              xx.lyx     -rw-------

(Same results when  launching 'dolphin' file manager as user2 to open a lyx 
file, through 'su'').