I already reported this bug in the past, but it’s getting too irritating and I think that it should be finally addressed. Certain objects (mostly small ones) automatically switch between visible and hidden isocurves while using some custom viewport whose isocurves are set to be hidden. I captured a video to show the issue (the video is still processed by YouTube and will be available in about a hour):
The file was downloaded from this thread:
Hi Bobi - Selected objects should always show the isocurves as set for the object as far as I know, regardless of the display mode setting. The discrepancy in zoom is a little peculiar - is that what you are reporting or the fact that isos show at all?
-Pascal
Hi Pascal, the issue is that the selected object’s isocurves won’t show up while the camera is too close, despite that they should appear all the time (if set to visible from the Properties).
Yeah, I cannot reproduce that here so far - can you export one of the display modes that shows this and post it so that at least I have the same starting point?
My guess of the moment is that the fact that there are much larger objects in the scene is messing the the depth testing of the isos when you get in close and there are objects well outside, and behind, the camera frustom,but that is a wild guess.
Ah @Rhino_Bulgaria - I can reproduce it in V7, I had V8 open before where it seems to be correct. And it does seem to be a problem where there are larger objects in the scene.
-Pascal
I’ve noticed this bug, subconsciously. But now it’s in my conscious lol and it’s irritating 
I wonder if it’s ‘camera frustum’ ‘highlight selection’ related…
Yeah, it only happens on small objects next to much bigger ones. For example, when I work on cars projects and have to zoom in to a screw or another small part, its isocurves also disappear. I usually hide all isocurves anyway, but sometimes I need to look at them while modeling NURBS surfaces to get a better idea about the shape. This is where the issue becomes irritating. A temporary “solution” that I found is to isolate the small object so that its isocurves become visible all the time.
Hi Bobi - if you are looking at V8 these days, can you open one of the files that causes problems in 8 ans see if it is any better in real life? Here, in my tests, it seems correct.
-Pascal
Hi Pascal, I just downloaded the latest Rhino 8 WIP and can’t reproduce the same bug there. Seems like it’s happening only in Rhino 7.
@pascal , while trying to reproduce the bug in Rhino 8 WIP that exists in Rhino 7, I actually found another bug related to the rendering of NURBS edges. I have custom viewport modes set to render the surface edges with 1 pixel thickness. However, at certain angles and especially at close-up the edge thickness switches to 2 pixels instead of 1 pixel as it should be.
The video showing the bug in action will be soon available here:
I remember now, that a fellow user didn’t like seeing the isocurves even during selection. So, my recommendation for them was to make the width 0, or something.
The bug is that the shown isocurves suddenly disappear upon close-up view. 
yeah I was going on a tangent sorry lol 
I’ve seen the bug you describe, but cant’ recreate it rn some reason.