Placing a bell or carpet directly on top of a mechanical piston facing upwards and then trying to move it will cause the bell/carpet to be removed without dropping any items.
If you instead place a linear chassis between the piston and the bell/carpet, moving it will cause the bell/carpet to break and drop its item whilst also being moved successfully. This ultimately causes the item to be duplicated.

I agree with the items not dropping being a bug, but the carpet duplication could be a vanilla bug that carried over in a slightly different form

Carpet duplication does work in 1.14 with regular pistons but the setup is a bit different. There must be a dead coral block below the carpet. I think in that case it's an issue that has to do with dead coral acting weird when you pull it.

Fixed in 0.2.4