When I went through the same thing with Emily, she would only go outside the box in the room where the box was
This is exactly what Luna is doing!! Sometimes she would even start scratching in her boxes and then go somewhere else! Maybe the litter or the box are too uncomfortable for them when they are in pain and they look for a softer surface?

I also noticed that Luna uses a box only once (that's why she has three boxes meanwhile). I was thinking that maybe the bacteria are giving her urine another scent, so she doesn't like to use a box that smells like that?

However, I had a big cleaning session in her room yesterday, and it seems most of the smell is gone now!

Kirsten