Before and after 17w47a, and before at least snapshot 18w16a, when you triggered a piston with a Marker-true armor stand on its head, the Marker-true armor stand fell through (as it should).
Old video also showcasing the old (correct) piston behaviour with Marker-true-AS in 17w46a and before:

Since at least 18w16a, this behaviour changed again: The Marker-true armor stand will not fall through anymore.

I couldn't test a couple of snapshots versions due to real life busyness, so I don't know when this behaviour started to occur again.

Old bugpost with the same bug which got fixed rightaway: MC-122255.