Same here, borrowing the neutral off the upstairs circuits, its common in pre mid 80's houses here, I warn people at board changes that if it's in their house that it's extra to correct. Last one was a gift thankfully, they were getting new heating system and immersion moved to garage, so plumber had traps lifted everywhere, even right over the pipe for the downstairs switch and the wall behind the landing switch was the old hot press in the bathroom and was open to be ply lined for tiling for a shower lol a tasty bonus since I'd priced it to correct, if there, beforehand. Lol