The problem is due to an unusual combination of factors. In quiz settings preferences, Submit All is turned on. On each quiz slide, or at least on some quiz slides, you have moved the Submit button off the slide completely. On those slides where the Submit button is not at least partly overlapping onto the quiz slide, the quiz slide will advance to the next slide when the slide duration completes. If you have the default 3 seconds slide duration, the quiz slide where the Submit button is outside the slide will automatically advance after 3 seconds.
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