Pushing End Rods only occasionally breaks Cactus

Cactus is happy to sit next to end rods

Set up a sticky piston to push an end rod beside a cactus, the cactus will not break, nor will it break when the piston retracts.

If however that end rod would push another block, (even a torch or something that will break when pushed) the cactus will break.

I assumed this was just that Block 36 broke cactus, but strangely pushing two end rods next to the cactus simultaneously will break it as well. I'm not sure it this is a separate bug, or if I've misdiagnosed this one.

I would have expected the end rods to never break the cactus, always break it, or break it only depending on facing.