Stationary Lights and Preview watermark

Can anyone PLEASE tell me why Lights defined as Stationary sometimes cause a ‘Preview’ watermark to appear in your renders?

What makes this especially frustrating is that I do not see the watermark in PIE mode!

You will get ‘preview’ when ever a light build is needed to correct the shading. Basically any time you move any object.

Much easier to make the level with moveable directional light, and switch back to static if you want it when you’re ready…