I installed libreoffice, and had Source Sans Pro set as my system font. After the installation, this font was deleted from my computer and I had to install the font again. I'm on windows 7 sp1.
I can confirm that upgrading to v4.3.3 from v4.3.2 using Custom Install causes the Source Sans Pro font to be deleted from the system. I did not test Typical Install. Tested on Win 7 SP1.
After a little more investigation, it appears that ALL the LO fonts were deleted, not only Source Sans Pro.
*** This bug has been marked as a duplicate of bug 76239 ***