Does that show in your original footage straight from
mini 3 ? I'm assuming it does not, what I see is a heavy export compression, also the waves approaching the shore are choppy which suggests that perhaps your editing software frame rate timeline is not matching the frame rate of the original clip ? In short, I'd made sure the frame rates are matching and increase bitrate (set to constant bitrate not auto) to 40 000kb (40Mbps) if exporting in 1080p, 100 000kbps if in 4k. Perhaps you've already figured that one out, but it does not look like
mini 3 issue to me. Hope that helps.