Iray convergence ratio and SSIM broken when canvases enabled

Saccharose03

Looks like canvases setting messes up the completion calculation of Iray progressive rendering, including the SSIM value and Iray convergence rates. I'm not sure if I need to submit a support ticket, so I came to the forum for help first. I did a simple test with Genesis 8 base female. canvases is set to the beauty pass of the full scene without alpha.

As you can see, when using Iray convergence rate to determine the completion of the rendering without canvases, it took 244 iterations (25.166s) to reach a 98.03% convergence rate. But when I added a canvas in advanced setting panel, it jumped to 451 iterations (43.414s) to reach a 95.65% convergence rate, almost double time to reach a similar convergence rate.

As for SSIM predict, things were even worse. Normally, it took 120 iterations (13.051s) to exceed 98.9% SSIM value when canvases was set to off. But when canvases was on, The SSIM value became abnormally high. it reached 98.9% in just 4 iterations (3.331s) while the image is full of noise, which is totally wrong and make Render Quality SSIM totally unusable.

According to my understanding, the beauty pass of the full scene without alpha is basically the same as the normal rendering, except it will save EXR file at the same time. Therefore the iteration needed to reach the same completion condiction should be similar. So is this a bug? I don't have any other software that uses Iray, so I can't tell this is caused by Daz Studio or Iray. This issue can be reproduced on Daz Studio 4.21.0.5 and 4.21.1.41 Public Beta. Logs are attached.