Blocks can disappear or be "duplicated" when they travel through different chunks.

Here, another piston head is created. But if it's the redstone block that moves between 2 chunks, it will sometimes disappear. These are not ghost blocks.

To reproduce, build that kind of clock, and unload and reload the chunks. You may get this. I got it several times at the same spot.

A possible important thing to say : these 2 chunks were in my spawn chunks before I changed them by /setworldspawn somewhere else. But they weren't the edges of spawn chunks, or lazy chunks. (This is happening after the change)