When a Shulker Box is closed, it acts like a solid block, allowing for a Redstone Signal to pass through. Upon opening the Shulker Box it should no longer be a solid block therefore not allowing for a Redstone Signal to pass through.

For versions lower than 1.20 (up to 1.19.4) this is true, as seen in this screenshot:

For Versions above 1.20 (1.20 & 1.20.1 at time of writing) this is not true:

Further explanation to what can be seen in the screenshot:

To properly update the Redstone Dust next to the Redstone Lamp, it is updated every tick using the "contraption" of Dropper, Hopper and Comparator. (In this case the Redstone Dust receives a Block Update every tick)

The player seen in the screenshot simply opens the Shulker Box, wich in versions lower than 1.20 unpowers the Redstone Lamp, since the Shulker Box is no longer a solid block.