Hi Francisco, thanks for the help.
I've been checking it for unwelded vertex, inverted edges, mesh corruption for the last 2 days, I pretty much remodeled everything like 3 times already, same thing happens. Also tried to export in different formats and bring it to a fresh file, with no success.
I guess I've expressed myself wrong, I'm not applying a smooth modifier, but using the auto smooth on the smoothing groups itself (after clean all), the modifier I need to apply is chamfer(quad), which is when the normal issue shows up. As soon as I set the tension to be 0,5 it gets all the normals issues.
I've tried turbosmooth after chamfering it myself, which looks much better. Or using the chamfer modifier with up to 4 segments (also looks good, still a triangulation issue here and there), but nothing that I can't fix with an edit poly over... but then I'll have problems with the size of the scene, since the project is a whole housing master plan and I'll go from 30k tris each "group of units" to 360k tris each.
I'm going kinda crazy with these one, it's an insanely simple mesh to give this much trouble.