Hi, grow from seeds will not have an extent over the whole segment by default, that is why you either paint the limits you want it to have on a dummy segment (outside the segment you want to split, it will not be taken as a seed since its outside the editable area but will still define the boundaries of GFS)
I see you only have one seeding segment, so it is normal for GFS to not work, you need to place seeds in the vertebra AND make another segment with seeds in the sacrum AND a segment outside the main one for limits.
In total these are your 4 segments:
a) original segment (The only hidden one)
b) seeds in the sacrum (painted with editable = inside original segment) (Visible)
c) seeds in the vertebra (painted with editable = inside original segment) (Visible)
d) seeds at the boundaries (top bottom left right front back) in a segment FULLY outside the original (you can use editable=everywhere or outside other segments) (Visible)
–> Then launch GFS with editable=inside original segment
Nb: you can bypass making d) by placing the seeds in b) and c) such as they cover the boundaries of the segment if that’s more convenient for you