A clip that has moved between apps several times looks unmistakably worse than the original. The degradation follows from how each platform treats what it receives.
Every upload is a re-encode
No platform stores what it is given. Each re-encodes uploads into its own formats, discarding information that its encoder judges unnecessary.
That judgment is made against the file presented, not against the original recording, so the second encoder sees the first one's output as the truth.
Each pass therefore starts from an already-degraded picture and removes more, which is why the loss compounds rather than accumulating gently.
Artifacts are preserved as if they were detail
Compression produces characteristic patterns: blocking in flat areas, ringing around edges and smearing in motion.
A later encoder cannot distinguish these from real detail. It spends bits reproducing them faithfully, which leaves fewer bits for the picture that matters.
The effect is worst in exactly the material short-form favors, since fast motion and rapid cuts already stress an encoder before any of this begins.
Watermarks make it worse
Downloaded clips often carry a moving watermark, which is high-contrast, animated content overlaid on the picture.
Moving elements are expensive to encode, so the watermark consumes bits that would otherwise go to the video, on every subsequent pass.
It also marks the clip as second-hand, and platforms generally treat visibly recycled uploads less favorably in distribution than native ones.
Cropping the watermark away does not help either, since removing it means enlarging the remaining frame and discarding part of the composition to hide a defect.
Resolution and frame rate mismatches add damage
Apps target different resolutions, aspect ratios and frame rates. A clip moved between them may be scaled and resampled at each step.
Frame rate conversion is particularly destructive because it invents or discards frames, producing uneven motion that no later encoder can repair.
Cropping to fit a different aspect ratio compounds the loss, since it enlarges an already degraded region of the frame.
The fix is upstream, not downstream
Sharpening or enhancement filters applied afterwards cannot restore discarded information; they emphasize what remains, artifacts included.
The only effective approach is to export a fresh copy from the original project for each destination, at that destination's preferred specification.
This costs a few minutes per platform and eliminates the entire problem, which is why it remains standard practice for anyone publishing the same material in multiple places.
Keeping the finished project available makes those exports trivial later, which is the practical argument for archiving projects rather than only the files that were published.