This issue might be more of an 5D3 issue than a lens one.
I did some night shots yesterday and just now noticed similar issues with my 5D3+24-105, it's almost not visible in the untouched raw file(I didn't notice it, until I accidentaly found it), but once I crank up the dials it becomes clearly visible.
I think it's visible all the time, but at least I couldn't tell until I dialed up almost all the development settings to the max, where I suddenly started to see a white circle in the frame.
Might not be an issue though, as I can't think of a scenario where I'd push the processing to the extend that I did in my tests (trying out new stuff). But if you search for it I'm sure you'll be able to find the white circle too.
Funny that I found this now.. :|