This is because LibreOffice is using SVG icons instead of PNG icons. ¿How to fix this little bug or bad behavior? Very easy! We need change the default icon theme used by LibreOffice (SVG) for others icons (PNG, ICO, JPEG...)
THE FIX
Go to http://www.deviantart.com/art/Libre-Office-Applications-Icons-478932157 and download a new LibreOffice applications icon theme suite. Remember, the new LibreOffice applications icon theme suite is not important because you can use anyone; the important is change the default icon theme used by LibreOffice. Unzip the new Libreoffice applications icon theme suite.
Then go to System > Preferences > Main Menu
In the right column, select "LibreOffice 5.3", do right click and select "Properties".
In the new open box, do left click on the default icon.
Then go to LibreOffice applications icon theme suite folder, select a new icon and open it.
Example:
Close the box. Do the same with LibreOffice 5.3 Base, LibreOffice 5.3 Calc, LibreOffice 5.3 Draw, LibreOffice 5.3 Impress, LibreOffice 5.3 Math and LibreOffice 5.3 Writer.
Close the box. Now go to Applications > Office and check your new submenu.
Now the MATE Desktop menu is fixed and go faster.
That's all.
