As LinkedIn on mobile is having more and more posts show only the first line, this type of view would be a nice addition.
I've examined how this works briefly on Android mobile app.
I'm pretty sure this is a bug in the app. iOS app seems to produce previews the same as the web (3 / 5 lines).
But it's been a few months now, and there hasn't been a fix yet. I guess I could give it another look.