You may have a look at my series of blog posts about Quizzes in the eLearning Community. Tried to explain everything including your issue.
https://elearning.adobe.com/2019/02/captivates-quizzes-2-submit-process/
https://elearning.adobe.com/2019/04/quiz-tweak-4-submit-process/
Look at the Timeline. Pausing point of the quiz slide is at 1.5secs. After the second step in the Submit process playhead is released AND the actions you specify for Success/Last Attempt are done. If you left those actions to the default 'Continue' command, the playhead needs to go through the last 1.5secs = 45 frames of the question slide before reaching the next slide.
Two possibilities=
- Drag the pausing point near the end of the quiz slide
- Change the actions to 'Go to Next Slide'.