dispensers seem to under no circumstance place shulker boxes.
similar to a bug in older versions, but it does it again, after being fixed back then.

reproduce:
place a dispenser with any kind of shulker box with any contents inside.
power it.
dispenser thinks it worked, but it just drops it.

EDIT:
i tried this in 1.12.2 and it DID place the shulker boxes.