I noticed that Octane has trouble in specific situation with light distribution between polygons that are placed on some large distance like > 5meters. It is much more noticeable on exterior architectural scenarios where skyligh should be bounced from ground and travel up to light surfaces that are not directly exposed to direct light. Under balconies or deeply corners, small tunnels we have light leaking - they are underlighting. Indirect light has problem to reach to these areas on larger distance.
This is why architectural oriented renderers like Corona or VRay do it better by default.
After some tests I found solution but I don't know that this is not a cheating, dirty solution. There is need to turn on Blackbody Emission and everything looks as in Corona or even better because we have control for emmission power. But other renderers magically do it with using only surface diffusion parameter.
I of course had albedo at max and cranked up sky power. Path Tracing diffuse depth on max.
Bad thing to use Emission is that while using dusk or night sky lighting scenarios we must turn it off or reduce because walls looks unrealistic. Two different materials for two different light scenarios ? Very bad. Also as I know Blackbody Emission should be used for surfaces that generate light.
Also this is why I thing that Octane architectural viz looks worse that these done on Corona or Vray - because people should turn on Emission to compensate underlighting.
First image is with albedo only on surface, next one is with Blackbody Emission on.