I've encountered the same preview vs. generated issues. I spent HOURS the other night, adding the blur effect to sensitive PII info for a video that is ~1hr 40min long. The preview looked great, then I exported, and the blurs were off in positioning. All of that work --- drained. I figured the preview was displaying a different aspect ratio than what I exported (both were default) and this lead to the difference in preview vs generated. That said -- thats a horrible usability issue. Your users need to know that the preview needs to be the same resolutions as your intended export.