Explanation
Saving a video from TikTok and uploading it to Reels carries TikTok's watermark, and Instagram has been explicit that such uploads are deprioritised. The fix is to export the clean file rather than re-save from the app, which is the whole of the problem and the whole of the solution.
A tool's own watermark is a different cost. It is the clearest possible signal that a post is an advertisement, on surfaces least tolerant of one, which is why watermark-free output on free tiers is a meaningful rather than cosmetic distinction.
The common mistake
Confusing a visible watermark with provenance metadata. Content Credentials are invisible and tamper-evident, and removing a visible mark does nothing to them.