@Micha in a backup copy of your CyclesFlicker2.3dm file could you remove the materials
Metall_sat and replace them with the materials in the zip FixedMaterials.zip (1.2 MB)
Assign to the objects and see if they work now better. At least here I can no longer reproduce the difference in tint when hiding the spring.
If this works then the problem is somehow related to the IOR setting. In your materials it is 1.0, whereas in my manually recreated materials it is 1.52.
I don’t know yet why this happens, but if this workaround gives you the results you are looking for you should be able to finish your animation without having to wait for a fix. I have currently no idea why the difference happens, so I have no ETA for a fix.
edit: I can’t redo in a file created from scratch, but it still happens in your file
Here from my file created from scratch two renders subtracted
RecreatedFromObjs.3dm (6.6 MB)
For the recreated file I had originally the HDR Mechanik environment texture in, but that was too big so I swapped it out for a different one. Still no differences between with and without spring though. The materials are the same as from FixedMaterials.zip (1.2 MB)