This has been happening to me on practically every keying project I work on. I send the sequence to After Effects for keying, complete the key, it looks great, return to Premiere Pro CC and every now and then a glitch or artifact appears which leads me to having to key that sequence again. Here a screen shot of what I'm talking about:
The footage is from a Canon C300 cut natively as an .mxf file. Keylight 1.2, Levels and MB Looks are used to create the final key.
Has anyone come across this before?
nest key then add MB and levels.
See if that will help.