This is a video issue.
Watch the tutorial. This shows how to set a poster frame in light room for a video.
Did you try to do this yourself? It doesn't make any difference for me. It only makes the poster frame work within LR.
Here is two screen shots. One is the lightroomn with poster frame set as last frame then exported to the new folder. You can see even as a thumbnail the poster frame did not change. Its stuck at about 7 seconds, whereby it was at the end before I made a change. I have the same functionality within AE, but nothing works.
Word from Adobe support:
" It is not possible for Video file Exports, It is possible for DVD or Blueray using Encore, where we can set End Action. "