My experience: when loading a save where the trader is already at the trading post, the trade wagon will always look like it’s approaching the trading post, AND will always notify you that the trader has arrived. I’m assuming in this case you had saved the game right as he was scheduled to leave, thus producing the results you see.
What @TheStranger93 says, it seems that a new routine has been introduced, the arrival procedure is repeated on load-in, whatever the trade-timer is for the visit.