I copied an existing face and made some edits to how the hands rotated. I then did a build and it worked for all watch platforms WearOS, old and new. I decided to change the artwork on the face, so I did an edit such that all I changed was the face art and hit Update. Would not build for WFF watches.
I just copied that and did another build for all WearOS types and the build succeeded for WFF. Loads on my Samsung Galaxy Watch 8 (Classic).
Just as well given the output from WFS runs badly. Exporting a .aab file and importing into Facer Creator was not a solution.