I shot in Slog 3 with an A7siii in a Cfast card, .mp4 footages, and i got those artefacts coming randomly, either in my timeline, in my exports. I tried convert in.mov the footages, doing a Prores export, nothing seems to work. i dont know how to get rid of it.
We're sorry about the poor experience. We're aware of a similar issue & are working on getting it addressed as soon as possible. In the meantime please try transcoding the source files using any external Transcoder like HandBrake & then import the transcoded file in Premiere Pro. Hope it helps.