^{Update by Mega_Spud:}
Summary:
Comparators measuring items in a shulker box, give out an incorrect signal.
A chest, chestcart, and a shulker box have 27 slots. Comparators are supposed to emit full signal when they are full, i.e. 27 stacks of items. However, for shulker box comparators emit full signal for only 27 items, not stacks.

Steps to Reproduce:

1. Place a chest and a shulker box
2. Place a comparator and a 15 long line of redstone dust from each comparator
3. Place 27 items in the chest, and 27 items in the shulker box

Observed Results:
The shulker box gives off a full redstone signal of 15, whereas the chest only outputs a signal strength of one.

Expected Results:
The shulker box should behave the same way as chests, and only give a full signal of 15 when all of the slots are full.

Screenshots/Videos attached: Yes

Notes:

Original Description:
When using a comparator to detect the amount of items in a chest, the comparator gives off a signal strength of 15 when the chest is full, however when using a comparator to detect the amount of items in a shulker box, the comparator gives off a signal strength of 15 when there are 27 or more items in the shulker box