The OP stated there are four (4) spotlights in the scene.

Attached is a scene I just rendered using three (3) spotlights.

There are lots of sharp corners, and despite IDL quality zero (0), there are no artefacts.

Cranking up IDL quality is only useful if there are nothing but light emitters used in the scene. If there are "real" lights, high IDL settings are just a waste of time.

So, with four spotlights, the reason for the artefacts must be something else.