The machine I’m testing this on is slightly lower spec than yours and I am not seeing the same behavior so I requested our Hardware team test this on a machine with the same hardware as yours and they, too, were not able to reproduce this behavior.
Please describe the exact conditions in which this is occurring. ie. If this occurs in a blank project, which template are you testing with. What are your preview rendering settings? Do you have any special Editor settings? etc. Is this still occurring in UE4.12.5?