Did you check the Matching Quiz Master slide after editing the default object styles?
Coincidence, but I double-checked this morning a similar problem for another user, linked with the Quick Start project Aspire. The developers forgot to edit the quizzing master slides. Matching question had white font on white background. It can be easily fixed by editing the default styles. If you do this before inserting the quiz slides, everything is updated correctly. If you do it after the quiz slides are inserted, you may need to check the master slides, and eventually use the button 'Reset Master slide' for the already inserted quiz slides.