Hi Pierre,
I use proportional notation and the new spacing section is never happy when combined with that, it appears.
Oftimes I just wish I could tell lilypond to stop being so smart and do note spacing how I want it, regardless of whether it is 'correct' or not. Horror of horrors, I would even like to be able to specify point by point spacing in linear units like ems! [As you can see, a hopeless case am I!]
Andrew