What are you export settings? Improving the codec and bitrate may lower the compression artifacts you are seeing.
this is only for the first few seconds then its clear with no artefacts.
I run Premiere Pro for some month and had never any issues but with the newer versions
the probs has just started.
The Moon pic is 1250 x 1250 i tried to export this in 720p and 1080 p with the same results.
The first few seconds you get square pixels and then you get a clear picture.
This had never happend before.
Be sure to avoid 2 pass, there's currently a bug with that. Use only 1 pass encoding.