As mentioned in MC-160953 when you use a piston and push a block into the world border the client thinks it went into the border but the server still thinks it is still in the world, or vice versa. But when you use the /setblock command to place a block in the glitched block, the textures overlap, causing it to appear strangely. I've tested this using a lot of blocks and they turn out to be the same effect. Blocks that are able to be interacted (e.g. doors, trapdoors, chests) can still be toggled/interacted. Pistons push the block placed using /setblock, resulting in the piston head being IN the giltched block.
Blocks with GUIs can still be used. Beds can be used too. In short, the game thinks everything is normal, except for the glitched block.