Every time you select landscape tool (mountain icon or Shift+3) in Unreal Editor viewports are set to real time rendering mode.
How to replicate:
Open empty project and create a new landscape. Go in any other mode than Landscape Mode. Set Viewport Option to not update viewport in real time. Now go to Landscape Mode again and the viewport options change to update viewports in real time.
No change in viewport settings when changing modes.
Landscape tool works fine in both real time and non real time editing. IF this viewport options change is not necessary band aid fix to some problem, it shouldn’t happen. Real time mode can cause problems in large scenes when development machine is not equipped with the most powerful GPU. Also it makes development machines emit more heat and noise which makes offices noisier than they need to be. I have personally always preferred to work in non real time mode in editor, only using real time when needed.
This has happened at least in in UE4 versions 4.12 to 4.15
So it appears this is not a bug at all since in the source code of the UE4 editor there are following lines at LandscapeEdMode.cpp line 466:
// Force real-time viewports. We'll back up the current viewport state so we can restore it when the
// user exits this mode.
const bool bWantRealTime = true;
const bool bRememberCurrentState = true;
So whatever the reason is it toggles itself on real time mode is unclear, but it seems behaviour is on purpose at least. Still a bit of a shame to have GPUs at 100% usage making office space very noisy for no real reason. Non realtime editing works just fine and doesn’t ramp up GPU fans at all.
I guess IF someone from epic is actually reading these reports I could turn this into a feature request: option to not turn on real time mode when sculpting landscape. Same thing when painting landscape.
As you have discovered this behavior is expected, and is done to make sure you can properly modify the landscape and use all the associated tools. If you would like you can simply press (Ctrl+R) to disable realtime quickly.