I guess you're using noise level as your render stop condition, right? Thing is that Corona calculates noise slightly differently with light vs dark background. Let's say you set noise limit to 5% and Corona reach 4,9% noise with light background, but 5,1% with dark background. That means that in the first case the render will stop after 5 passes, but in the second case it needs to continue till it reach set target and since by default the noise level is calculated after each 5 passes, Corona will render those additional passes even if that means that your image now has something like 3% noise. Long story short - it's not the bug. If you need more consistent render times, use other render stopping conditions, like pass, or time limit.