I hesitate to pipe up, but my experience with fluffy sativas indoors is that they really don't like to be topped or trained. I've been growing some fast veg, single cola ones and they stand tall and straight with stalks like fiberglass. When I've tried to train, they seem to end up with too many branches to fill out, or something like that.
Floppy. It's startling to see them grow naturally. So I'm wondering if it has to do with calcium uptake in early bloom - maybe can't get enough to grow a lot of stalks?
Which brings me to my point. How's the calcium load in your soil? It would probably be helpful to make sure that there's enough. Maybe heavier on calcium based nutes too?