This is unlikely to be a fault with the camera or lens. The most likely causes are scene lighting or reflections creating a prismatic effect.

From the examples described, possibilities include:

• a reflective object (such as a watch crystal) catching your flash and bouncing light into or around the scene
• another surface in the room acting like a prism and splitting the light
• colored LED/stage lighting producing rainbow bands along shadow edges

A clue is that the rainbow appears within the scene rather than as a uniform image defect. If it was caused by the sensor or lens itself, you’d expect it to be more repeatable across shots in similar conditions. If it came from LED lighting, it may also be visible to the eye and appear as color separation near shadow boundaries.

The UV filter is a possible contributor to extra reflections/ghosting, though probably not the main cause of the rainbow pattern. Removing the filter is a good troubleshooting step, especially when using flash in reflective environments.

So: first suspect the lighting and reflective surfaces in the room, then test again without the UV filter before blaming the lens or camera.