I always set all clips to Full after the XML import. I did this also in Resolve 12.
When grading a new project without the stills from 12, it works for me this way:
Setting the Data-Levels of the imported clips in Resolve to “Full”, grade them and then (for the roundtrip) set the format to ProRes422 and under the advanced settings set the Data-Level to “Video”.
Now the files look the same in both programs.