Why a caption can be visible in the file but hidden in Reels
Instagram draws profile information, the post description, audio details, and interaction controls over the uploaded video. Text underneath those overlays still exists but becomes difficult or impossible to read.
Use the center as a conservative starting area
Keep important text away from the extreme top, bottom, left, and right. The center-to-lower-middle often works when it does not cover a face or focal object.
Safe placement changes with the shot
A single fixed position may cover a speaker in one cut and a product in another. Move individual lines when the subject changes position.
Burned-in placement stays fixed
When captions are rendered into the MP4, Instagram does not restyle or move them. The app can still overlay its interface, which is why placement must be checked before posting.