With the introduction of selection boxes more closely matching the collision boxes, it appears that there has been an oversight when implementing these changes for the (extended) piston.
That appears to be working fine, but when you look at the part of the shaft that belongs to the base block (as far as occupied block space is concerned), it does not in fact register as selection and you are able to place blocks "through" the piston:
(Note the selection box on the grass block below the piston)
I assume that the extended piston's base only has the actual base assigned as collision box as well, but I wasn't able to fit an entity item through there as that is too large and collides with the head.
The ideal solution would probably be to have the entire extended piston appear selected when looking at the base, as cutting of the box of the shaft where the block space ends would appear weird.