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@Troy_S is correct on his comments I will elaborate a bit.
If you are adding an element with alpha channel over the hole of the same element on the background, created by another alpha channel, you will get those edges. The images are being added but the alpha channels are not combined.
You can try to denoise any image or image sequence with the compositor, without denoising data. The result will obviously not be as great as if it was using denoising data, but you won't have to re-render everything.
Ok here is a simple solution go to your collection and move the ground from background collection to foreground collection
and the other way is to disable holdout for the background you will find it in the filters in collection