I had this issue on a couple of my computers a few months ago, it got fixed and now its back again 
Apple Mail app preview lines not visible.
Toggling between side and bottom preview doesn’t help.
Changing to non or any number of preview lines doesn’t work either.
Tried with various combination of quitting an starting mail again.
Reboot Mac doesn’t help either.
In the past I even nuked and paved the entire Mail Library and preferences witch also did not help. Over time the preview lines came back.
This morning they were there, out of nowhere the lines disappeared again.
Apple stop messing things up