Not only weld.
You used autogroup and I bet that in your figure some groups touch each other, athought theirs bones are not next in chain. For example chest group touch shoulder (right or left) group, but in the middle should be collar (right or left)

Quote - > Quote - Only a guess but ...

It would appear that the vertices are not "weld"ed where the crack occurs.

cheers and Happy Christmas,
dr geep
;=]

That's my guess too ;o).

Laurie