The manual says:
Display Quantize Exceptions
→ To edit a display quantize event, double-click its flag, adjust the settings in the Display Quantize dialog, and click Apply.
→ To remove a display quantize change, click its marker to select it and press Backspace or Delete , or use the Erase tool.
(Cubase_Pro_Score_12_Score_Layout_and_Printing_en.pdf — WebHelp Version)
Which is it? Quantize flag or Quantize marker?