I have a problem, entirely of my own making, which is causing family members to die as they come of age.

I slightly messed with the campaign files to give the Vale of Anduin an Éothéod Royal Family with new names and cavalry bodyguards. Now whenever Framgar son of Fulgar comes of age, depite having been with us all for 17 turns. Framgar is the younger brother of Frumgar and so this shouldn't be related to any heir triggers. Framgar was also aged 12 at the start of the game so he was created from the descr strat file and shows up correctly in the family tree along with his mother Cynwise and siblings. Frumgar and Folca.

Does anyone have any thoughts about his post-infant mortality rate of 100%?