This might have been covered elsewhere in the past (RDNA, SM Poser forums) but I can't remember reading about it, so if it's already been explained/explored then forgive me (and please post the answer!).

Here's a very basic scene:

This is a cube primitive set to white, a plane/floor set to RGB 127,127,127, and a sphere primitive whose material is simply a Superfly Emission node set to white and Strength 500. (I know that's quite strong but it's necessary to show the issue.) Those three elements will remain the same throughout.

Now I add bagginsbill's envsphere prop.

In the Material Room for the envsphere I've simply added a Superfly Emission node to a CyclesSurface, attached the image map (in this case, "shady_patch_4k.hdr" downloaded from HDRI Haven), and set the Emission Strength to 1.

The issue might already be visible. It is, of course, how much dimmer the light from the sphere primitive has become. It might be argued that the light from the HDRI is flooding the scene and making the sphere primitive's light appear dimmer. Which is a fair argument, I suppose.

To test this, I tried lowering the Strength of the Emission node on the envsphere. I lowered it to 0.1.

At one tenth of the original light, I'm still surprised that the primitive sphere's light is (potentially) being overwhelmed so I try reducing the envsphere even further. This time to 0.01

Okay, I'm now starting to think this is getting silly, so I try one more test. This time, I've set the Emission Strength to 0.00001 meaning there should be almost no light from the HDRI in the scene except for any extremes.

At this point, the light from the sphere primitive ought to be illuminating the scene in the same way as the original image, with only the tiniest contribution from the HDRI. But clearly it's not. For some reason, when the HDRI is contributing even the smallest amount of light, the other emissive material is barely doing anything. (Notice the grain in the final image compared to the first image; a sure symptom of low light conditions.)

If I reduce the Emission node's Strength to 0 on the envsphere, the result is an image indistinguishable from the very first image (i.e., the primitive sphere's light is back to its original strength), so it's not merely the presence of the envsphere that's causing this issue (it's still there just not contributing any light).

I've tried this twice now just in case I'd done something to produce this effect the first time around. The second set of renders was done from a fresh restart of Poser.

Why does this happen? Is it only me (I could easily be doing something wrong here and totally not seeing it!)? Is it something particular to bagginsbill's envsphere? (My first guess is "No, why would it be?" but I'm open to persuasion.) Or if it's a Superfly/Cycles thing then why is it happening? (And, more importantly, how can it be fixed?)