Exported Project has random pixelation

Hi folks any help appreciated with this..

I am delighted to have finished my film. I have been doing various exports as I finalised the final version the past few weeks, with absolutely no problems up until today.

Today the exports are resulting in a file that has random areas of pixelation.
Even though it's perfect in the timeline. I'm exporting optimised media to H264

What I've tried:
1: I deleted render files -exported again - didn't work.
2: I created a new project and copied and pasted into the timeline - exported again - didn't work.
3: I created a new library and project and copied and pasted my timeline into that new project, (the media was stored in the library so it made copies of that for the new library).. That actually solved the pixelation, but for some reason it did not copy the colour and effects properly, so it gives me a new problem!

Update!
4: I exported as ProRes which gives me way too big a file at 50GB..but.. there's no pixelation.

I wonder what is going on? And how I can get a file under 10GB that has no pixelation.

Try using Apple Devices with HEVC or Compressor.