Well, yes, as Koopster said, it WAS broken. To be honest, I never proved by myself why it was, but indeed, I saw a lot of people complaining about it and stuff. As Telinc said in
this post, it seemed to be related to the "per-level sprites" feature and some Asar bugs (which as you can read, are all fixed now).
The horizon looks clearer with GIEPY, I must say. It's not a coincidence that I decided to give a new oportunity to smw hacking just because this tool is almost done and better in almost every aspect to the previous sprite tools. Taking advantage of the words, I must thank to the current ASM team, which (I'm sure) they're working hard to make this change an actual improvement to the smw hacking, and not a disarray which was PIXI.