In my case at least, after the team at DxO had analysed my raw files and failed to reproduce the symptoms I went back to Apple with this issue.
After some investigation, it was determined that the reason I was having this issue was due to the set up I have. I have my Photos library on a mac server, and this is not supported. The funny thing is that I have been running this set-up successfully with no issues previously. The problem triggered only after MacOS Sierra.
Moving the photos library to my local machine fixed the issue I was having - however any edits I had done previously all have to be redone to prevent them from showing pixelated, and to allow them to be exported as JPEG/TIFF files.
Of course having to redo this work and also not being able to share my photo library easily (without using the cloud) are both far from ideal - but at least i can say the fault lies with Apple and not DxO.