WORKAROUND:
1) Click on the "Export to Graphic File" icon in the toolbar
2) Select the PNG File Type
3) Set the Scale to at least 300 (100 makes the image resolution too low in most cases for the text to be clearly readable when printing)
4) Go to the PNG tab and set pixel format to 32 bits
5) Click OK
6) Open the generated .png and print using desired page settings. The symbols on the diagram will stick in place now.
The generated image file should now have a higher resolution and should be readable when printing.
STATUS:
Waiting for fix in a future release of Toad for Oracle.