Currently, when burn-ins are enabled on level sequence actors, they always show in the viewport, regardless of whether it's actually playing or not. This is mostly due to the complexity of engine and slate ticking, and capturing frames from the render thread to ensure we always have a burnin on the first and last frame.
However, when using the just for debugging purposes, this makes them unwieldy. We should consider making them more flexible and exposing their visibility to blueprint so users can show/hide them as they deem fit.
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-43177 in the post.