OK SO
This is gonna be mad annoying but recheck every "placed" box.
Some of the preset placed boxes are NOT actually placed, I discovered it after going through 60 or so boxes

There's one in the vent in the bathroom.

I will say this though... You can miss one box and still get 100% on the furniture score at the end. This happened to me because I thought it would be fun to try and yeet the vent box further into the vent.

I ended up with one piece of furniture missing, but since there is a decent bit over 100 pieces it rounded up and gave me 100%.

I had a glitch and the box ended up in the installed box. I found it by manually rearranging the boxes - the lost one just jumped out at my feet.

One of the boxes is askew just lying on the steady structure right from the beginning.

had this happen to me, if you pick up one of the already placed boxes it adds one but never goes away, if your 100% sure youve placed them all just finish the mission and you be fine

Omg yes I found it thank you. After manually rearranging them I found it on top of the other boxes . SMH

I was missing the box in the bedroom on top of the light. Just look at the ceiling and you'll see it