When going up and down, the choice of initial position on the branch is arbitrary. By default, the first position is picked initially, and then Symex the squirrel remembers where it was on each branch after that, so that you return to your last position when going up and down the tree. If you’d like to move to the first or last position at any point, you could use (for instance) M-h or M-l at each level, as usual, or traverse the tree using f and b, instead.
If, on the other hand, you’d like to start always at the first position when going up (as it was in older versions of Symex), disable the branch memory feature by adding this to the :custom section (not the :config section) of youruse-package form:
(symex-remember-branch-position-p nil)