Well, that one is still 720 x 480 and not 1280 x 720, so something is resizing it (not Corona).
Never render straight to a video format for one thing, always render to a sequence of stills and then composite to video afterward (e.g. using the free Davinci Resolve, if you don't have a video editing app already and would like a free one). If rendering to video, you are usually introducing video compression and you may not like the amount of compression, which would mean re-rendering - with a sequence of stills, just adjust the compression / quality level in your video app and re-encode, no need to re-render. And if there's a power outage or a crash, your animation format file is destroyed and you have to begin at the start, but with sequence of stills you just start from the frame you left off.
Anyway, we still can't tell you what is happening because we don't see the 1280 x 720 output version to see what came out of Corona (untouched by video compression), so we still can't advise on what is introducing the effect you describe as we don't know what is doing the resizing as well as what is doing the video compression.