Pistons won't push/pull a rail with the block it sits on

Normally when a piston pushes or pulls a rail from one solid supporting block to another, it does not break the rail. But in this example, where the supporting block is also part of its load, it does, as in the example attached. This happens even if the block under the rail isn't a slime block, as long as slime blocks are used to carry both on the same piston.