After finishing cloth painting, the ability to toggle Realtime Rendering in the viewport is greyed out. Additionally, the Disable Realtime Override button doesn't seem to have any effect. This problem can be resolved by restarting the editor.
Tested in //UE4/Release-4.26 @ CL 15973114 and this occurred there as well, so not marking as a Regression.
Realtime Rendering cannot be toggled
After cloth painting, the ability to toggle Realtime Rendering would not be affected
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-120627 in the post.