You tried to simulate colored paper by changing white to grey?
It is better to leave that alone (unless you are going to print on colored paper).
Use a rectangle filled with a color instead and put it on a layer below your other layers.
I want to avoid using a colored rectangle because I want to export the content of a large amount of artboards at once and I need a transparent background when exporting.
I used the same method in another document as well and there it works perfectly.
It is easy to hide the rectangle.
Select same fill color; change to none or hide selection.
Such change is natural. White color in terms of printing is an absence of any inks. Actually, it's a paper color. So, if you change the paper color (= artboard color), your white lines become paper-colored lines.
So, the best way to preview your artwork on a colored background is to follow Ton's advice (draw the colored rectangle) and then hide it before export.
You can draw the rectangle on a separate layer, copy and paste it to all artboards (Ctrl/Cmd+Alt/Opt+Shift+V). Then before exporting hide or delete this layer with all rectangles.
But if you want to print the white lines on the colored surface, you have to use the proper Pantone ink (e.g. PANTONE 11-0601 TCX. Bright White)
This is incredibly weird. Yesterday I checked and my old files that I had changed the Artboard in were still working normally.
Now I opened it again and white is invisible on every document now. Adobe had to have changed something, because I surely didn't.
Without seeing your actual files, nobody can tell you what you did differently in those other files.
White on paper color always worked like it does now.