Observer facing shulker block emits 2 quick redstone pulse upon opening/closing it. First version containing this bug: 19w13a (snapshot for 1.14).
Tested on the latest version 1.14.4-pre5, and most versions since 19w13a.
There's no blockstate change when opening the container. Traditional Block Update Detectors do not detect opening shulker boxes, only observers
A bug that got fixed. See MC-107664. The observer being triggered twice upon opening and closing is MC-160467.