Most People Never Reach Your Ending
Autoplay is silent and the feed is crowded. People are scrolling fast, and your ad has a couple of seconds to earn the next couple of seconds.
I had a client hit 500,000 video views where only 3% watched the whole thing. Sit with that for a second. If the entire point of the ad was saved for the last frame, 97% of those people never got the message.
The drop-off is not a flaw in your editing. It is just how people behave on a phone.