Lightbug on a rotating mesh

Hello boys and girls :slight_smile:

Can somebody explain me this bug? I have a simple rotating mesh a Spotlight above. I build the light and press simulate. The model seems to be dark and then bright. Normaly i think it must be bright if the parts are under the spotlight. But it changes.

[video]https://youtu.be/M0wj5clZ7c4[/video]

THX