DL vs PT, huge changes that shouldn't happen Tweet

[boxfx]

Can anyone help me explain the following:

When using Direct lighting, adding a soft, low power light to the background of my scene completely obliterates all the foreground lighting. When using Pathtracing, this does not happen.

C4D project attached
Octane 2020.2.5, GF3090, C4D 20

[boxfx]

Increasing the sampling rate of the smaller white light to 100 fixes it, but why? I thought sampling rate was there to increase render quality for tiny bright lights which might otherwise get missed by the sampling algorithm?

My instinct is that its the large size of the blue light 'stealing' all the samples, but if you make the blue light smaller, but brighter, the same problem occurs