This was an odd one. It had me flumoxed for a while and i ended up creating the view a couple of times trying to figure out what was going on. I of course went to “Reveal hiddee elements” but that did not show the section lines either.
Scale and section lines
It seems when you go to a smaller view size, say from 1:100 to 1:1000 there is a setting in Revit that will allow you to hide some of the Annotation based on a preset that you can choose.
So you have a plan with sections shown at 1:100. You duplicate that view and make the new view scale 1:100, the section lines disappear.
In the properties, locate the parameter “hide at scales coarser than” when selecting a section line. But of course, in your current view that is not there, so you have to go to another plan and select it there then tyou can see: