Ok, I'll try to explain without getting too lengthy, plus I'll post a couple photos of the issue.
I recently got a new Sony A7iii. I noticed that very occassionally the footage would have some artifacting in it. After changing memory cards didn't help I contacted Sony, but they don't want to assume responsibility because when the clips are played back in the camera the artifacting is NOT there. So in other words, it seems to be something on either the comptuter end or Premiere end. Computer by the way is 2018 iMac.
Anyway, the problem is getting worse, but, I've noticed 3 points that I'll mention here:
1) I have the camera set to record to both memory cards at the same time. While the artifacting does not show up when playing back in the camera, when the artifacting does occur it is on BOTH memory cards. (Thus it would appear that the issue IS with the camera, but that's not the point now).
2) I'm not sure I can blame Premiere on this because it does appear that the artifacting is on the memory cards, even when viewing the files straight on the desktop and not in Premiere. But as per my next point, Premiere is now doing something wonky...
*3) Despite 1 and 2 above, here's the biggy. The artifacting issue is occuring more and more, but now it's occurring randomly on footage that didn't have it before. And, sometimes if I put an effect on the clip with the artifacting, or change the speed, or even just nest it, then the artifacting either goes away or gets worse. So it now seems that it has to be something within Premiere because a clip can play perfectly fine, then later have the artifacting issue.
The second part of #3 above is that now the artifacting is occurring upon export! See the attached photos. These two photos are a screenshot of the video playing in YouTube after being exported (LEFT) and the same clip in Premiere (RIGHT). In Premiere NO artifacting, Exported file HAS artifacting. And if I export the file again, as I've done a few times now, the artifacting will occur somewhere else. Even over jpeg photos now as is the case in the second pic so now it's not even happening with only the A7iii footage!
Sorry not to make this post shorter, didn't know how else to explain it all.