To make it clear, this is not about zero-ticking pistons (some people would complain if that were to be removed, but that's of course up to Mojang's discretion).

If you power a piston normally (not zero-ticking) which got another piston on top of itself, an entity on top of the second piston will fall through it.

Once the entity has fallen through the second piston, you can warp the entity through that second piston back on top of it.

Not part of this bugpost (yet?), but an entity does not only fall through pistons (which are sort of a special case in some aspects anyway); by powering a piston below a block normally, you can have that entity fall through a solid block as well.

Both happen, as soon as a block is above the entity.

Personally, I can see interesting gameplay benefits for entities to fall through also solid blocks, but I don't think this is intended.
Is there maybe a bugpost for that already?
In any case, had to add this to be more precise and complete in this bugpost.

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TLDR: If an entity is located on the same block as a piston, upon piston retraction that entity gets warped in front of that piston; if the piston is in an upright position, this means of course it is warped on top of it.

How to reproduce:
You can use a /setblock-command in order to get both entity and armor stand on the same space

but here a simple example for a Survival method:

Stack a piston on top of another piston and an armor stand on top, and a block above that armor stand, then trigger the lower piston.
(You don't have to shortcut a piston by powering it directly with a button, you can simply power it normally, but I wanted to showcase that piston-specific behaviour.)

Upon retraction, the entity will fall through the upper piston.
Trigger the upper piston, and the entity gets warped on top of it.

Sorry if I missed an already-existing bugpost, I only know and found about the one that was closed back in the day.