Hey, first thx for the reply !
I looked at my build scale but everything is set correctly. It seem there something wrong in the import/export.
If I'm importing a heightmap File (scale set at Raw) and then exporting it (using the same Node for Export), I will get slightly modification which can only be fix by Gamma correction.
In fact my expected behavior would be that if I'm not doing any modification to the import file, it should export the same 1:1 which it don't.
Export setting set also at Raw, EXR seem the closest to initial file but still not quite the same