I haven't changed any of the default processes, all I did was change the look of the next button.
The reason I am using the next button is because every slide on the project includes a back and next buttons so for consistency.
I have created a new quiz slide and keep it exactly as it is apart from expanding the correct answer feedback to overlap with the next button, hopefully that will explain it better
Edit view:

Runtime view:

Got it, and know cause. Feedback captions or shapes will always appear on top of everything, that is also the case for buttons, shape buttons, click boxes! Now I understand what is happening: you replaced the feedback caption by that image and that is causing the issue. The Next button is not acting as a Next button, but it is clicking on the slide (second step of the submit process) that is advancing the slide, not the Next button.
I hope this explains what I mean? There is a way to have images appear, but don't replace the feedback because you'll always have this issue. Did you read the blog post?