It is weird that they did not find anything to be wrong with the seat but I don't think they deliberately sold you a malfunctioning one. Was the business cabin full so you could not switch to other seat?

I had a BKK-IST flight where there was equipment change. I had originally been seated alone in row 4 window seat and when they re-issued tickets at airport gate I asked why I was moved when the plane still have that same seat (not that it matters, they tend to clear all and re-issue them first come, first served basis it seems). I also asked was there any seat left where I could sit alone like I would in original plane.

They told me that there is row 4 aisle seat that I could sit alone if I wanted since the window seat in that row have mechanical problem and so I took that. The problem with the seat was that it would not recline.

So, atleast in this case they knew about the problem and were not willing to sell or seat anyone into that seat.