Regardless of your ‘auto play’ setting, Amazon Prime Video displays a “Next Up” box bottom right, with movie artwork, on reaching the credit offset point; because, after spending perhaps more than two hours watching a film, who doesn’t want immediately want to start another?
Here’s the adblock rule you need:
www.amazon.co.uk##DIV[class="nextUpWrapper"]
(i.e. you’re blocking div.nextUpWrapper)