Curves are made sure that the 2nd CV is aligned to the 1st. I’ve repeatedly snapped them and they seem fine.
Still, no go.
So, I remember that if the 2nd CV is normal to the mirroring plane, I only need tangency to get curvature across the surfaces.
Still no go.
As a last ditch attempt, I duplicate the curves off the failed surface, set up support surfaces and create a new square.
Again, no go.
Anyway, this was an exercise and not really needed for my final model. I’m going back to modelling 1/2 of it instead. Still weird, though.
EDIT: Going really overkill and making sure the 2nd and 3rd CVs were aligned in both corner curves, then duplicating these to form the other corner and building a square with implied tangency(!) from those did produce a proper curvature aligned mirrored surface… but man, why couldn’t it just work from the start? 