there are different things you can do: try using the debug menu. The combination F3+A for example reloads all chunks. If any debug doesn't help, try finding out what type of item this seat is. Is it an actual block, or something loose like an item frame? If the latter is the case, using the command /kill @e will kill everyone in the server, so you will lose xp, but it also removes minecarts, rails, item frames, etcetera. It might help as a last resort. You could also try to use
/setblock [coördinates of bugged block] minecraft:air 0 replacethis works better but requires cheating. it will turn this block into air. If this doesn't work, there are millions of other ways to remove blocks, try some. Any of these things, especially the last one, always helped for me in case of bugged blocks, try them out!