ok i think i found a way !
On the left, the height in TIFF format directly saved from Alchemist. We can clearly see the layered effect.
On the right, the same image opened in Photoshop, then saved in TIFF with NO compression.
Result is much much better !

Zbrush doc says hat alphas should be saved as uncompressed tiffs of PSD for clean Result.
All my png tests were bad. but uncompressed TIFF is working perfectly.
Maybe in the future if a checkbox can be added in Alchemist, but it's not a big deal.
Thanks again for your help !