Don't spoil the ending.
You've probably seen creative video ads that are interesting at first. Still, in the end, something happens that is entirely unrelated to the main plot but aimed only at advertising. For example, the action took place on the street, and the plot led to the ending. Still, at the end of the video, the blogger appears in entirely different clothes at home and says something like: "By the way, my followers can get a discount in the company X ... ".
Such video ads came from Instagram, where they work, though causing adverse reactions. On TikTok, however, nobody will watch such an ad. If you created an 18-second video, and no one is watching its last 3 seconds because of the ad, it won't gain many views. This is how algorithms in TikTok work, and there is nothing you can do about it. Therefore, the entire video should look like one whole.