If You place a gravity affected block like sand on top of a shulker (even with NoAI:1) the block is going to instantly break into item form.
It should probably stay in a block form and, just like all other blocks, make the shulker teleport (if he has NoAI:0). Also when a falling block falls on top of a closed shulker it shouldn't break, since shulker is a solid entity and fills completely 1 block of space. Block breaks even for summoned falling blocks that are blocks that are not affected by gravity, for example stone.
Used Command:

/summon falling_block ~ ~ ~ {BlockState:{Name:"minecraft:stone"},Time:1}

If we assume that shulkers are not full blocks than this behavior is still not consistent with other not full blocks (for example trapdoors).