I've created an island and like you do wanted to plop some trees on it.
I followed the H17 Master class on Heightfields but got different results.
When I plug simple geo into the scatter all is good. When I plug in some outside geo of a very simple tree it kind of explodes.
I modelled the tree in Modo and made sure all was centred and merged.
Any help on this would be fabulous
https://we.tl/t-XohcDiUT9n [we.tl]
(link to hiplc file)