I hope this had not been reported yet (not a duplicate). Tested in 1.8 and 1.7 environments, the problem can be reproducible by the following step.
1. Add questions in one category.
2. Then add those questions (either random or selective mode) in a quiz.
2a. (Optional step) remove those questions from the quiz.
3. Next, rename the categories of those questions.
4. Add the questions back to a quiz using random question.