When using a custom formation with a 6-character party, the 6th character will not abide by the position assigned to it.
SAVED GAME: link
REPRO STEPS:
Load the attached saved game (if I did things right, you'll be in Caed Nua with a 6-party character + Itumaak).
Observe that a custom formation is currently selected; check how it looks "on paper" (i.e. how I arranged characters in the custom formation interface).
Move the party.
While characters move, observe that Sagani's and Itumaak's green pointers (the green circles that appear on the ground where the characters will stop) are not where they should be according to the custom formation assigned.
When characters get to the point where you clicked, further observe how Sagani and Itumaak are in completely different positions from what the custom formation would mandate.