Still QC related. Basically, they were powered by a block 2 blocks away, so when those blocks are destroyed, there is no update sent about it to the pistons. If you were to place a block on one, it would update, notice it isn't powered. Then the update would cascade and all of them would retract.

All the pistons are powered by so called quasi-connectivity. When activating one of them directly, the rest get updated and activate too. QC is one of the least intuitive features in redstone.

Original design of the farm by
The Archivists: www.youtube.com/watch?v=s7JZ...

I tested it on 1.20 (2b2t) and 1.21 paper (other SMP). Works without the armor stands.