Noise Level Limit bug

[Daskydesign]

Hello,
I started to use a FairSaas license this month and I have a big problem.
I'm trying to finish a project:
I'have a very simple scene with:
- shot as "Direct visibility override"
- Hdri as Environment Map
- a plane with shadowCatcherMtl and same shot as Enviro/Backplate
- a simple Camera
- an object

I set Corona in a "Noise level limit" and It is very unstable.
Some render work well and others take a long time (almost infinite time)

It reached in very little time 0% noise level (but is very noisy) and does not stop up to endless passes.
Sometimes I stop and re-start render and work fine.
Sometimes this reaches 0% and does not stop after a short time, the noise level increases again (for example 24%) and does not stop.

I am attaching a screenshot of a render that I had to stop because I was going to infinity (noise limit set at 1.5%)

You can understand that it is impossible to work well with this problem.

[Ondra]

this is probably caused by the noise level being calculated from entire image, not just foreground - so the HDRI with 0% noise influences it. We will probably rethink this in 1.7

[Daskydesign]

Okay but, why sometimes work and sometimes not work in the same frame without changes?

[Ondra]

it should be consistent in exactly the same frame - can you send us the scene where it is not so we can test it?

[pokoy]

I noticed recently that sampling with adaptivity works differently on a white vs a black background. Can't say if this happens here but if yes this may lead to a different noise level with the same pass number on another frame.