Used a slime block to push an armour stand up and have a piston push under it but the armour stand just got stuck in the activated piston.
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I don't know their setup, I only fiddled with AS (also pistons) in the past, also in combination, see e.g. MC-122255 and MC-129144.
There are ways to get a Marker-False-AS (thus Survival-AS) into a piston (, and there's some other partially implemented-however-not-intended-behaviour that I bumped into whilst checking for this bugpost here, but the Devs might not be very open towards touching the piston code just yet again).
I just quickly tested by summoning AS, but additionally, I also used very basic Survival builds to confirm that the Survival-AS can fall through.
TLDR: I can confirm that Survival-AS can also fall through pistons, and you don't necessarily need slimeblocks for that, however, I don't know if this is what OP means/experienced.
Please provide a screenshot of the setup before activation, please also have the F3 debug screen enabled.