I've been modding this model to use in my animations, and thought it would be cool to add an emission shader to the pupils of the eyes (the model is a robot). I was fine with just having the pupils glow, though I do plan on trying to find a way to condense the light into a laserbeam type style later on, but when I added an emission node (not from the principled BSDF), it would not show up at all in the render. Annoyingly, this problem is the same for both Eevee and Cycles:
Even weirder, as seen by the picture above, the emission clearly is working in the material preview. but look at the picture below, where I go into the rendered preview:
It does not render. Obviously, this is not just the preview I'm talking about; I'm getting the same results when I hit F12. Like I said, whether it be Cycles or Eevee, I'm still getting the same results, so I do not believe that the render engine is the problem here. Thinking that it might have just been a problem of that specific material on my eye model here, I opened up a new .blend file with just the default cube (no lights). To that, I added an emission node and turned up the strength to 1000000 (which is, of course, useless, but it effectively demonstrates my point), and checked the rendered preview:
Same results for both Eevee and Cycles. As one more test, I tried just simply turning on the emission in the Principled BSDF, and was surprised to see that the emission was suddenly working again:
Not only that, but it was working just as it should in both render engines (showing up in Eevee renders, and actually illuminating surrounding objects in Cycles). This led me to believe that there is a problem with my node setup, not the render engines or anything else. And I suppose that makes sense; I feel like I'm missing something from my material that is supposed to make the emission work, but I'm not sure what it is. I checked the normals for my mesh, too, but nothing was flipped. I'm not sure what else to do. Can someone please help me with this?