When the Global Illumination scalability settings are changed in the editor from High to Medium, they primarily act on the r.Lumen.DiffuseIndirect.Allow and r.SkyLight.RealTimeReflectionCapture. Normally, when r.Lumen.DiffuseIndirect.Allow is changed, the render states are re-created, however, when there is a Landscape the OnChanged callback is hijacked and FGlobalComponentRecreateRenderStateContext is no longer called. In addition, the render states need to be refreshed after r.SkyLight.RealTimeReflectionCapture has been updated, but the order of the settings in BaseScalability.ini has the Lumen diffuse indirect changing first and then the SkyLight CVar. Rather than changing the order, FGlobalComponentRecreateRenderStateContext can be used in the OnChangeSkylightRealTimeReflectionCapture.
Expected
The indirect lighting on the landscape returns to the same as it was when scalability was previously set to High
Actual
The indirect lighting partially stays at the Medium or Low levels
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-225380 in the post.