Is there some reason the 1/8 value should be missing from the quantize setting drop down list ? I work in score view. Its possible to go to key mode and set the quantize to eighths, but this should not be necessary, unless I am missing something…
It’s not missing on mine but it is out of order. My list has it at the bottom. I’d love to change it but not yet found a way! 
Accidentally i erased the 1/16 quantize preset so i fixed it modifying the RAMPreset file.
Go to the Presets folder inside Cubase Mani folder (Programs/Steinberg/Cubase/Presets).
You’ll find a file called RAMPresets.xml.
Open it with notepad and do the necessary corrections (add,remove or correct order).
Maybe you need to Google to recover the original RAMPreset file and do a simply copy paste of lost quantize presets.
You even can add new quantize presets from other grooves like the MPC or Logic or Cakewalk this way.
Cheers