I'm using V.12 Yeti which has been upgraded to 12.0q. I actually upgraded just a few minutes ago due to this puzzling problem. I happends in both V.12p and the upgrade.
Here is the deal: I model of a simple tire with just a few splines. Since I want to decal in cylindrical fashion, I made a horizontal and vertical marker. Problem is, they don't show up. I changed views, and some views show the markers, but the front view has nada. Then I selected the model, and as soon I had a bounding box around the model, the markers appear. Deselect the model and the markers disappear again.
So I saved the file and closed down AM. After reopening AM and the file, nothing changed. Still no marker settings. Now I changed from OpenGL to DirectX and back. No difference. Ok, so now I turn off everything and reboot my computer. Guess what, no diff, still no marker lines.
Then I copied the tire model (basically just a few circle splines extruded) and created a new project file. Pasted the tire model and the same problem occurs. Well, last resort - I upgraded to V.12Q. Still no difference.
I suppose someone is going to say, why don't you just start the model over again in a new project. Yeah, I could and then have no answer to this puzzle when it happens to a model that has 1000 cps.