OK @mekkablue, thanks to you I can now understand now why things have changed :
As I’m working on an Uppercase font, I was designing only caps.
Everything was fine then whith the variable height when tested in Adobe Apps.
A few days ago, I duplicated the characters to create lowercases.
That’s why the baseline problem now occurs, besause of what you said about the lowercase d.
So I took a look at your Vertical Metrics Tutorial, but I have to say that — even in my native language ! — I d’on’t undesrtand anything about the hhea or sTypo thing 
…
Is there any specific nformation I have to know to solve my problem ?
(I don’t want the baseline to be offset when slinding variable height…)