Exports up to 50 MB are saved to Assets. A larger export is still made: you see "Saved locally, but not to Assets" followed by the reason, and the Your video is ready window still opens.
What to do
Tap Download MP4 in the Your video is ready window to save the video to your device, then post it from there.
To keep a copy in Assets so you can attach it to a piece and use it in Publish, shorten the video by removing scenes or lowering each scene's Sec value, then tap Export MP4 again.