Steps:
Open some project in the 4.12
Select the tab of place at left top and drag and drop something to your scene.
Then check the realtime to turn off in the viewport
Go to the paint tab for example top left then the viewport turn realtime another time
Uncheck the realtime viewport to turn off
Go back to Place tab
Then go back to paint
As you can notice the realtime viewport is another time turn on, and turn on too in others tabs at change even if turn off all, that is the problem.