I realize that there isn't much to go on yet. I'll update this report if I catch it happening. But I've noticed that occasionally, my Sim will use a toilet in one restroom, and will then go to a sink...
Ideas
I noticed this too very recently, it was fine before and it has been fine for a long time, I would since they released the update to choose if its a kitchen or bathroom sink... I have set them appropriately and now sometime my sims will use one sink that is much far away from them.
I noticed its like the in The Sims 1, where if you were going to a community lot, sims all around in the lot would gather to the place where the camera/the player is looking at, to make the game look busy... it reminds me of that because each time I realized sims were washing their hands in a sink that is far away, its because I look at that sink, while I sent them previously to the toilet, closest to where they were... unless the sink is set to kitchen of course.
