Found that out after running git fsck on a repo where I've been using go-git (through git-bug):

$ git fsck
erreur dans l'objet tree ac67188617b481887f55414af55b02631deaf5b9 : treeNotSorted: not properly sorted

Turns out that tree entries need to be sorted when serialized. Beware thought, it's a slightly peculiar order: isomorphic-git/isomorphic-git#937

I would assume that it would be go-git's reponsibility to ensure valid stored data.