Page 61 of the new Glorantha Sourcebook concretes "fewer and fewer whole trolls born" with attempts to counter the curse resulting in litters, in RQ:G as originating from Gbaji in powers he gave to the broken council. Two curses, one at the dragonewts and one at Kigor Litor and the Dark Trolls.
The Trolls Curse is in their children (the curse of kin).
The dragonewt curse failed, but (speculation on my part) may have been "devoured by the dragons" and (potential plot hook) diverted into the creation of the black egg seen in the scenario "The Ruin on the Stream".
I've got 2 trollpaks (2&3) but barely opened them so it may be hidden in there somewhere.