I loaded my pre-endgame save to play Beast of Winter. To prep I went to Nekataka, and following trying to leave the game gives me a letter received prompt (which I presume is the trigger for the DLC) but the game then kicks me immediately into the endgame sailing choices sequence prior to reaching Ukaizo. Reading the letter versus storing it in the stash/trying to leave on foot versus by sea make no difference. Anyone found a cause/workaround to this?

Maybe don't go to Neketaka? Go somewhere else? Other than that, load a previous save.