If you place 64 or more tile entities (such like signs, furnaces, chests,...) in the same chunk by using the /fill command wherever it replaces another tile entity, it still displays the former tile entity. Sometimes it displays the tile entity you have placed and the former tile entity in it.
This is just a render bug. You can use every single block you've placed with /fill still as the block you've placed and not the one that you see. If you reload the map, everything is fine again.

This is probably related to MC-82703