SOLVED: There are several AUR packages for Hl-L3270CDW.
The problems were happening with brother-hll3270cdw . It’s PPD it’s defective, it seems.
Use instead brother-hl-l3270cdw, this one works fine. Both in alignement and in duplex printing (and everything else, fortunately).
Driverless is just OK if you don’t use dulex printing, but this won’t work properly.