Explanation
Each platform puts its furniture in different places, so a caption sitting clear on TikTok can end up under a Reels button, and the right-hand column of action buttons is wider on some surfaces than others. There is no single safe zone that satisfies all three, which is why the usual approach is to keep text well inside a conservative central band.
The consequence of getting it wrong is not subtle. Subtitles obscured by a share button make a video unwatchable on mute, which is how most of the audience is watching.
The common mistake
Checking the safe zone in the editor rather than in the app. The interface overlay is not present in your export, so the problem is invisible until it is published.